[5.4] warn when duplicate subform fields are removed on save

administrator/components/com_fields/src/Model/FieldModel.php

@@ -111,12 +111,30 @@ public function __construct($config = [], ?MVCFactoryInterface $factory = null)
      */
     public function save($data)
     {
-        $field = null;
+        $field                     = null;
+        $hasDuplicateSubformFields = false;
 
         if (isset($data['id']) && $data['id']) {
             $field = $this->getItem($data['id']);
         }
 
+        $fieldType = $data['type'] ?? ($field->type ?? null);
+
+        if ($fieldType === 'subform' && isset($data['fieldparams']['options']) && \is_array($data['fieldparams']['options'])) {
+            $seenCustomFields = [];
+
+            foreach ($data['fieldparams']['options'] as $option) {
+                $customField = (string) ($option['customfield'] ?? '');
+
+                if (isset($seenCustomFields[$customField])) {
+                    $hasDuplicateSubformFields = true;
+                    break;
+                }
+
+                $seenCustomFields[$customField] = true;
+            }
+        }
+
         if (isset($data['params']['searchindex'])) {
             if (\is_null($field)) {
                 if ($data['params']['searchindex'] > 0) {
@@ -172,6 +190,10 @@ public function save($data)
             return false;
         }
 
+        if ($hasDuplicateSubformFields) {
+            Factory::getApplication()->enqueueMessage(Text::_('COM_FIELDS_FIELD_SUBFORM_DUPLICATE_FIELDS_REMOVED'), 'warning');
+        }
+
         // Save the assigned categories into #__fields_categories
         $db = $this->getDatabase();
         $id = (int) $this->getState('field.id');

administrator/language/en-GB/com_fields.ini

@@ -69,6 +69,7 @@ COM_FIELDS_FIELD_SHOWLABEL_LABEL="Label"
 COM_FIELDS_FIELD_SHOWON_DESC="Conditionally show or hide the field depending on the value of other fields."
 COM_FIELDS_FIELD_SHOWON_LABEL="Showon Attribute"
 COM_FIELDS_FIELD_SMARTSEARCHOPTIONS_HEADING="Smart Search"
+COM_FIELDS_FIELD_SUBFORM_DUPLICATE_FIELDS_REMOVED="Duplicate subform fields were removed. Each field can only be used once."
 COM_FIELDS_FIELD_SUFFIX_LABEL="Suffix"
 COM_FIELDS_FIELD_TYPE_LABEL="Type"
 COM_FIELDS_FIELD_USE_GLOBAL="Use settings from Plugin"