Remove last duplicate, but not all duplicates

Hi,
I've tried the code and I've faced an issue which is in connection with deleting duplicate tags.
On the demo site I tried the Basic Settings.
When I typed "tags", bot of them were removed. I think the right behavior is delete only the last duplicate not all of them.
This same behavior occurs in my project.
Has anyone ever met one like this? If yes, is there any fix about this bug?