Fix duplicate icon add in export

SharedObserver did not check for already added icons add during export
leading to duplicate icons in the target db.

tests/TestMerge.h

@@ -57,6 +57,7 @@ private slots:
     void testUpdateGroupLocation();
     void testMergeAndSync();
     void testMergeCustomIcons();
+    void testMergeDuplicateCustomIcons();
     void testMetadata();
     void testDeletedEntry();
     void testDeletedGroup();