deleting duplicate pictures

Hi to all,

I do not find an easy way to delete duplicate pictures.

(I do use digikam 8.7.0 in German language)

I collect all pictures from every storage to store that on one NAS and 
than run the "find duplicates" (only 100% equals). During the progress I 
saw that some pictures have1 or up to 6 duplicates, finally it finds 
230.000 pieces. That is too much to do this manually, I think.

On main screen there is a button "delete duplicates", but it is grayed out.

What I am searching for is a batch program that deletes every duplicate 
but keeps one of the original pictures in place.

What do I make wrong or what can I do to solve my problem?