It sounds like you using the
Add Files function correctly. I don't think there is any way to block files from being scanned into the library using this method so...
Could it be that the files are actually already in your library?
• Maybe in the wrong node (e.g. Inbox or Audiobooks).
• Maybe mis-tagged somehow and you simply can't find them.
• Maybe you have a filter activated that is preventing them from being displayed.
I think the next step (which will check all these) would be to use the
Computer node (this is basically a folder browser tool found in the
Navigator Panel) to navigate to one of these files. The right-click context menu contains a '
Send To' sub-menu which looks a little like this:

The four entries highlighted are the four nodes of MusicBee where the file might reside. If any one of them is greyed out then that is where the file is located. You can click on
Music Library to send it there if that is what you want.
If none of these options are greyed out (as in the image here) then the file is not in your library and I/we will have to think again.
If the Music Library option is greyed out, then the file is already in your library and you can use the
Search > Locate In Library option from the same right-click context menu to find it.