diff --git a/lib/classes/databox/field.php b/lib/classes/databox/field.php
index 048dbee215..bba7f1b6c4 100644
--- a/lib/classes/databox/field.php
+++ b/lib/classes/databox/field.php
@@ -522,11 +522,22 @@ class databox_field implements cache_cacheableInterface
 
     public function set_dces_element(databox_Field_DCESAbstract $DCES_element = null)
     {
+        $connbas = $this->get_connection();
+
+        if (null !== $DCES_element) {
+            $sql = 'UPDATE metadatas_structure
+               SET dces_element = null WHERE dces_element = :dces_element';
+
+            $stmt = $connbas->prepare($sql);
+            $stmt->execute(array(
+                ':dces_element' => $DCES_element->get_label()
+            ));
+            $stmt->closeCursor();
+        }
+
         $sql = 'UPDATE metadatas_structure
               SET dces_element = :dces_element WHERE id = :id';
 
-        $connbas = $this->get_connection();
-
         $stmt = $connbas->prepare($sql);
         $stmt->execute(array(
             ':dces_element' => $DCES_element ? $DCES_element->get_label() : null
diff --git a/lib/conf.d/bases_structure.xml b/lib/conf.d/bases_structure.xml
index d4773a7717..071b5b7098 100644
--- a/lib/conf.d/bases_structure.xml
+++ b/lib/conf.d/bases_structure.xml
@@ -4720,6 +4720,13 @@
               <field>name</field>
             </fields>
           </index>
+          <index>
+            <name>dces_element</name>
+            <type>UNIQUE</type>
+            <fields>
+              <field>dces_element</field>
+            </fields>
+          </index>
           <index>
             <name>indexable</name>
             <type>INDEX</type>