"Append Mode" for bulk asset editor is storing duplicate values

Hello All,

  I am using AEM 6.3 with SP2.

  I wanted to edit the metadata properties (basically to add new values for a multi-valued property) of multiple assets at once.

I followed this document the https://helpx.adobe.com/in/experience-manager/6-3/assets/using/managing-multiple-assets.html  and try to add new values for the metadata with "Append mode" option checked. It's working perfectly fine.

But consider the following scenario.

1) I have uploaded 50 assets and added a custom metadata property lets say (material:diamond)to them

2) I add 100 more assets in same folder , and select all 150 and open in metadata editor screen.

3) I add the same property value which was added to first 50 assets (material: diamond).

4) I check the "Append mode" option and click save.

Now 

the first 50 assets are having duplicate values (material[]:diamond,diamond)

and next 100 assets are having correct values (no duplicates)

I don't want to have duplicate values for the same property and still i want to use  "Append mode" option because i have other available options(values) also which can be appended to the same  property (ex: material : diamond,gold,silver).

Please provide your suggestions, how to handle this scenario.

Thanks in Advance,

Rohit