I reported an issue (version 1.0) in the following post : https://forum.literatureandlatte.com/t/back-to-older-version-of-project/15682/1
This problem might be related to what I discover now, although now it is in version 1.0.2.
Each file dropped on corkboard duplicates in the previous file or folder shown on the corkboard.
The workaround is to import into any empty folder (I use Research and Trash for this) and then to move the file to its right place.
Importing onto the binder or in outliner does NOT duplicate the files.