I searched for commands and forum but did not found answer to this.
Is there a way for duplicacy to list for me files that are indeed duplicated in my backup so I can try to clean my source?
Not with Duplicacy directly. The output of the check command can give you an indication of how much de-duplication went on between backup revisions and repositories.
Personally, I use Everything on WIndows (be sure to get the 1.5 Alpha version via the forum) and using the dupe: dmdupe: sizedupe: etc. commands to spot redundant data in an instant.
Thanks. I thought it might be built in as this kind of info obviously is there 
Keep in mind, Duplicacy doesn’t target files when de-duplicating data - it doesn’t really know anything about that at the time of backup. It de-duplicates chunks, which might come from different files and different repositories on different computers.
Locating whole files that are identical isn’t something Duplicacy ever needs to do. (Although it’d sure be nice to do a reverse-lookup on a chunk and see which files it references.)