Prevent references fields from creating duplicates

term_merge-prevent_duplicate_references-2924108-2.patch

diff --git a/src/TermMerger.php b/src/TermMerger.php
index 2b341f6..d96ae0d 100644
--- a/src/TermMerger.php
+++ b/src/TermMerger.php
@@ -177,13 +177,30 @@ class TermMerger implements TermMergerInterface {
         }
 
         $referenceUpdated = FALSE;
-        foreach ($values as &$value) {
+        $targetTermExists = FALSE;
+
+        // Check if the toTerm already exists in the field values.
+        foreach ($values as $value) {
+          if ($value['target_id'] === $toTerm->id()) {
+            $targetTermExists = TRUE;
+          }
+        }
+
+        foreach ($values as $delta => &$value) {
           if ($value['target_id'] !== $fromTerm->id()) {
             continue;
           }
+          elseif ($targetTermExists) {
+            // Remove the fromTerm reference, if the toTerm already exists
+            // in the multi-value field. It will prevent duplicates of the
+            // toTerm.
+            unset($values[$delta]);
+          }
+          else {
+            $value['target_id'] = $toTerm->id();
+          }
 
           $referenceUpdated = TRUE;
-          $value['target_id'] = $toTerm->id();
         }
 
         if (!$referenceUpdated) {