diff --git a/plugins/woocommerce/changelog/fix-37660 b/plugins/woocommerce/changelog/fix-37660
new file mode 100644
index 00000000000..19479c9b9bf
--- /dev/null
+++ b/plugins/woocommerce/changelog/fix-37660
@@ -0,0 +1,4 @@
+Significance: patch
+Type: fix
+
+Use first meta value for HPOS migration when there are duplicates for flat column.
diff --git a/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostMetaToOrderMetaMigrator.php b/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostMetaToOrderMetaMigrator.php
index a10720d2b4f..85a8ea5555f 100644
--- a/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostMetaToOrderMetaMigrator.php
+++ b/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostMetaToOrderMetaMigrator.php
@@ -52,6 +52,7 @@ class PostMetaToOrderMetaMigrator extends MetaToMetaTableMigrator {
 				'meta'          => array(
 					'table_name'        => $wpdb->postmeta,
 					'entity_id_column'  => 'post_id',
+					'meta_id_column'    => 'meta_id',
 					'meta_key_column'   => 'meta_key',
 					'meta_value_column' => 'meta_value',
 				),
diff --git a/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderAddressTableMigrator.php b/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderAddressTableMigrator.php
index a9aba4bf6dd..526bd0a7da0 100644
--- a/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderAddressTableMigrator.php
+++ b/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderAddressTableMigrator.php
@@ -56,6 +56,7 @@ class PostToOrderAddressTableMigrator extends MetaToCustomTableMigrator {
 				),
 				'meta'   => array(
 					'table_name'        => $wpdb->postmeta,
+					'meta_id_column'    => 'meta_id',
 					'meta_key_column'   => 'meta_key',
 					'meta_value_column' => 'meta_value',
 					'entity_id_column'  => 'post_id',
diff --git a/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderOpTableMigrator.php b/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderOpTableMigrator.php
index 05f22f6d1d0..995540baaf8 100644
--- a/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderOpTableMigrator.php
+++ b/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderOpTableMigrator.php
@@ -40,6 +40,7 @@ class PostToOrderOpTableMigrator extends MetaToCustomTableMigrator {
 				),
 				'meta'   => array(
 					'table_name'        => $wpdb->postmeta,
+					'meta_id_column'    => 'meta_id',
 					'meta_key_column'   => 'meta_key',
 					'meta_value_column' => 'meta_value',
 					'entity_id_column'  => 'post_id',
diff --git a/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderTableMigrator.php b/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderTableMigrator.php
index e21085c62ba..8635eb36f7b 100644
--- a/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderTableMigrator.php
+++ b/plugins/woocommerce/src/Database/Migrations/CustomOrderTable/PostToOrderTableMigrator.php
@@ -39,6 +39,7 @@ class PostToOrderTableMigrator extends MetaToCustomTableMigrator {
 				),
 				'meta'   => array(
 					'table_name'        => $wpdb->postmeta,
+					'meta_id_column'    => 'meta_id',
 					'meta_key_column'   => 'meta_key',
 					'meta_value_column' => 'meta_value',
 					'entity_id_column'  => 'post_id',
diff --git a/plugins/woocommerce/src/Database/Migrations/MetaToCustomTableMigrator.php b/plugins/woocommerce/src/Database/Migrations/MetaToCustomTableMigrator.php
index 71b0d3022a4..55c3e9d1973 100644
--- a/plugins/woocommerce/src/Database/Migrations/MetaToCustomTableMigrator.php
+++ b/plugins/woocommerce/src/Database/Migrations/MetaToCustomTableMigrator.php
@@ -543,7 +543,11 @@ WHERE
 	private function processs_and_sanitize_meta_data( array &$sanitized_entity_data, array &$error_records, array $meta_data ): void {
 		foreach ( $meta_data as $datum ) {
 			$column_schema = $this->meta_column_mapping[ $datum->meta_key ];
-			$value         = $this->validate_data( $datum->meta_value, $column_schema['type'] );
+			if ( isset( $sanitized_entity_data[ $datum->entity_id ][ $column_schema['destination'] ] ) ) {
+				// We pick only the first meta if there are duplicates for a flat column, to be consistent with WP core behavior in handing duplicate meta which are marked as unique.
+				continue;
+			}
+			$value = $this->validate_data( $datum->meta_value, $column_schema['type'] );
 			if ( is_wp_error( $value ) ) {
 				$error_records[ $datum->entity_id ][ $column_schema['destination'] ] = "{$value->get_error_code()}: {$value->get_error_message()}";
 			} else {
@@ -610,7 +614,7 @@ WHERE
 		$query = $this->build_verification_query( $source_ids );
 		// phpcs:ignore WordPress.DB.PreparedSQL.NotPrepared -- $query should already be prepared.
 		$results = $wpdb->get_results( $query, ARRAY_A );
-
+		$results = $this->fill_source_metadata( $results, $source_ids );
 		return $this->verify_data( $results );
 	}
 
@@ -623,19 +627,13 @@ WHERE
 	 */
 	protected function build_verification_query( $source_ids ) {
 		$source_table                  = $this->schema_config['source']['entity']['table_name'];
-		$meta_table                    = $this->schema_config['source']['meta']['table_name'];
 		$destination_table             = $this->schema_config['destination']['table_name'];
-		$meta_entity_id_column         = $this->schema_config['source']['meta']['entity_id_column'];
-		$meta_key_column               = $this->schema_config['source']['meta']['meta_key_column'];
-		$meta_value_column             = $this->schema_config['source']['meta']['meta_value_column'];
 		$destination_source_rel_column = $this->schema_config['destination']['source_rel_column'];
 		$source_destination_rel_column = $this->schema_config['source']['entity']['destination_rel_column'];
-		$source_meta_rel_column        = $this->schema_config['source']['entity']['meta_rel_column'];
 
 		$source_destination_join_clause = "$destination_table ON $destination_table.$destination_source_rel_column = $source_table.$source_destination_rel_column";
 
 		$meta_select_clauses        = array();
-		$meta_join_clauses          = array();
 		$source_select_clauses      = array();
 		$destination_select_clauses = array();
 
@@ -646,31 +644,85 @@ WHERE
 		}
 
 		foreach ( $this->meta_column_mapping as $meta_key => $schema ) {
-			$meta_table_alias             = "meta_source_{$schema['destination']}";
-			$meta_select_clauses[]        = "$meta_table_alias.$meta_value_column AS $meta_table_alias";
-			$meta_join_clauses[]          = "
-$meta_table $meta_table_alias ON
-	$meta_table_alias.$meta_entity_id_column = $source_table.$source_meta_rel_column AND
-	$meta_table_alias.$meta_key_column = '$meta_key'
-";
 			$destination_select_clauses[] = "$destination_table.{$schema['destination']} as {$destination_table}_{$schema['destination']}";
 		}
 
 		$select_clause = implode( ', ', array_merge( $source_select_clauses, $meta_select_clauses, $destination_select_clauses ) );
 
-		$meta_join_clause = implode( ' LEFT JOIN ', $meta_join_clauses );
-
 		$where_clause = $this->get_where_clause_for_verification( $source_ids );
 
 		return "
 SELECT $select_clause
 FROM $source_table
     LEFT JOIN $source_destination_join_clause
-    LEFT JOIN $meta_join_clause
 WHERE $where_clause
 ";
 	}
 
+	/**
+	 * Fill source metadata for given IDs for verification. This will return filled data in following format:
+	 * [
+	 *    {
+	 *      $source_table_$source_column: $value,
+	 *      ...,
+	 *      $destination_table_$destination_column: $value,
+	 *      ...
+	 *      meta_source_{$destination_column_name1}: $meta_value,
+	 *      ...
+	 *    },
+	 *   ...
+	 * ]
+	 *
+	 * @param array $results    Entity data from both source and destination table.
+	 * @param array $source_ids List of source IDs.
+	 *
+	 * @return array Filled $results param with source metadata.
+	 */
+	private function fill_source_metadata( $results, $source_ids ) {
+		global $wpdb;
+		$meta_table            = $this->schema_config['source']['meta']['table_name'];
+		$meta_entity_id_column = $this->schema_config['source']['meta']['entity_id_column'];
+		$meta_key_column       = $this->schema_config['source']['meta']['meta_key_column'];
+		$meta_value_column     = $this->schema_config['source']['meta']['meta_value_column'];
+		$meta_id_column        = $this->schema_config['source']['meta']['meta_id_column'];
+		$meta_columns          = array_keys( $this->meta_column_mapping );
+
+		$meta_columns_placeholder = implode( ', ', array_fill( 0, count( $meta_columns ), '%s' ) );
+		$source_ids_placeholder   = implode( ', ', array_fill( 0, count( $source_ids ), '%d' ) );
+
+		$query = $wpdb->prepare(
+		// phpcs:disable WordPress.DB.PreparedSQL.InterpolatedNotPrepared, WordPress.DB.PreparedSQLPlaceholders.UnfinishedPrepare
+			"SELECT $meta_entity_id_column as entity_id, $meta_key_column as meta_key, $meta_value_column as meta_value
+			FROM $meta_table
+			WHERE $meta_entity_id_column IN ($source_ids_placeholder)
+			AND $meta_key_column IN ($meta_columns_placeholder)
+			ORDER BY $meta_id_column ASC",
+			array_merge( $source_ids, $meta_columns )
+		);
+		//phpcs:enable
+
+		// phpcs:ignore WordPress.DB.PreparedSQL.NotPrepared
+		$meta_data            = $wpdb->get_results( $query, ARRAY_A );
+		$source_metadata_rows = array();
+		foreach ( $meta_data as $meta_datum ) {
+			if ( ! isset( $source_metadata_rows[ $meta_datum['entity_id'] ] ) ) {
+				$source_metadata_rows[ $meta_datum['entity_id'] ] = array();
+			}
+			$destination_column = $this->meta_column_mapping[ $meta_datum['meta_key'] ]['destination'];
+			$alias              = "meta_source_{$destination_column}";
+			if ( isset( $source_metadata_rows[ $meta_datum['entity_id'] ][ $alias ] ) ) {
+				// Only process first value, duplicate values mapping to flat columns are ignored to be consistent with WP core.
+				continue;
+			}
+			$source_metadata_rows[ $meta_datum['entity_id'] ][ $alias ] = $meta_datum['meta_value'];
+		}
+		foreach ( $results as $index => $result_row ) {
+			$source_id         = $result_row[ $this->schema_config['source']['entity']['table_name'] . '_' . $this->schema_config['source']['entity']['primary_key'] ];
+			$results[ $index ] = array_merge( $result_row, ( $source_metadata_rows[ $source_id ] ?? array() ) );
+		}
+		return $results;
+	}
+
 	/**
 	 * Helper function to generate where clause for fetching data for verification.
 	 *
@@ -777,6 +829,9 @@ WHERE $where_clause
 	 * @return array Processed row.
 	 */
 	private function pre_process_row( $row, $schema, $alias, $destination_alias ) {
+		if ( ! isset( $row[ $alias ] ) ) {
+			$row[ $alias ] = $this->get_type_defaults( $schema['type'] );
+		}
 		if ( in_array( $schema['type'], array( 'int', 'decimal' ), true ) ) {
 			if ( '' === $row[ $alias ] || null === $row[ $alias ] ) {
 				$row[ $alias ] = 0; // $wpdb->prepare forces empty values to 0.
@@ -798,10 +853,6 @@ WHERE $where_clause
 				$row[ $destination_alias ] = null;
 			}
 		}
-		if ( is_null( $row[ $alias ] ) ) {
-			$row[ $alias ] = $this->get_type_defaults( $schema['type'] );
-		}
-
 		return $row;
 	}
 
diff --git a/plugins/woocommerce/tests/php/src/Database/Migrations/CustomOrderTable/PostsToOrdersMigrationControllerTest.php b/plugins/woocommerce/tests/php/src/Database/Migrations/CustomOrderTable/PostsToOrdersMigrationControllerTest.php
index 739d518724f..072125136fb 100644
--- a/plugins/woocommerce/tests/php/src/Database/Migrations/CustomOrderTable/PostsToOrdersMigrationControllerTest.php
+++ b/plugins/woocommerce/tests/php/src/Database/Migrations/CustomOrderTable/PostsToOrdersMigrationControllerTest.php
@@ -747,6 +747,34 @@ WHERE order_id = {$order_id} AND meta_key = 'non_unique_key_1' AND meta_value in
 		$this->assertEmpty( $errors );
 	}
 
+	/**
+	 * @testDox When there are mutli meta values for a supposed unique meta key, the first one is picked.
+	 */
+	public function test_first_value_is_picked_when_multi_value() {
+		global $wpdb;
+		$order              = wc_get_order( OrderHelper::create_complex_wp_post_order() );
+		$original_order_key = $order->get_order_key();
+
+		$this->assertNotEmpty( $original_order_key );
+
+		// Add a second order key.
+		add_post_meta( $order->get_id(), '_order_key', 'second_order_key_should_be_ignored' );
+
+		$this->sut->migrate_order( $order->get_id() );
+
+		$migrated_order_key = $wpdb->get_var(
+			$wpdb->prepare(
+				"SELECT order_key FROM {$wpdb->prefix}wc_order_operational_data WHERE order_id = %d",
+				$order->get_id()
+			)
+		);
+
+		$this->assertEquals( $original_order_key, $migrated_order_key );
+
+		$errors = $this->sut->verify_migrated_orders( array( $order->get_id() ) );
+		$this->assertEmpty( $errors );
+	}
+
 	/**
 	 * @testDox Test migration for multiple null order_key meta value.
 	 */
@@ -760,7 +788,6 @@ WHERE order_id = {$order_id} AND meta_key = 'non_unique_key_1' AND meta_value in
 		$this->sut->migrate_order( $order2->get_id() );
 
 		$errors = $this->sut->verify_migrated_orders( array( $order1->get_id(), $order2->get_id() ) );
-
 		$this->assertEmpty( $errors );
 	}
 }