log warning in console if duplicate ids are detected.

Author: botandrose-machine

src/idiomorph.js

@@ -1182,6 +1182,9 @@ var Idiomorph = (function () {
       for (const id of duplicateIds) {
         persistentIds.delete(id);
       }
+      if (duplicateIds.size) {
+        console.warn("[Warning] duplicate ids found during morph, state loss within these elements is possible:", Array.from(duplicateIds));
+      }
       return persistentIds;
     }