diff --git a/src/classes/entities/class-tainacan-item-metadata-entity.php b/src/classes/entities/class-tainacan-item-metadata-entity.php
index 13927969f..0130b6fa0 100644
--- a/src/classes/entities/class-tainacan-item-metadata-entity.php
+++ b/src/classes/entities/class-tainacan-item-metadata-entity.php
@@ -486,10 +486,35 @@ class Item_Metadata_Entity extends Entity {
                 
                 // if its required, at least one must be filled
                 $one_filled = false;
-                $valid = true;
-                foreach($value as $val) {
-                    if (!empty($val))
-                        $one_filled = true;
+								$valid = true;
+								$dupe_array = array();
+								foreach($value as $val) {
+									if (!empty($val))
+										$one_filled = true;
+									
+									if ($this->is_collection_key()) {
+										if (++$dupe_array[$val] > 1) {
+											$this->add_error( 'key_exists', sprintf( __('%s is a collection key and there is another item with the same value', 'tainacan'), $metadatum->get_name() ) );
+											return false;
+										}
+										
+										$Tainacan_Items = \Tainacan\Repositories\Items::get_instance();
+										$test = $Tainacan_Items->fetch([
+											'meta_query' => [
+												[
+													'key'   => $this->metadatum->get_id(),
+													'value' => $val
+												]
+											],
+											'post__not_in' => [$item->get_id()]
+										], $item->get_collection());
+				
+										if ($test->have_posts()) {
+												// translators: %s = metadatum name. ex: Register ID is a collection key and there is another item with the same value
+											$this->add_error( 'key_exists', sprintf( __('%s is a collection key and there is another item with the same value', 'tainacan'), $metadatum->get_name() ) );
+											return false;
+										}
+									}
                 }
                 
                 if ($this->is_required() && !$one_filled) {
@@ -505,7 +530,7 @@ class Item_Metadata_Entity extends Entity {
                 }
                 
                 $this->set_as_valid();
-                return true;   
+                return true;
             } else {
                 // translators: %s = metadatum name. ex: Title is invalid
                 $this->add_error( 'invalid', sprintf( __('%s is invalid', 'tainacan'), $metadatum->get_name() ) );
@@ -540,7 +565,7 @@ class Item_Metadata_Entity extends Entity {
             }
 
             $this->set_as_valid();
-            return true;   
+            return true;
         }   
     }
 }
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