If I'm downloading one torrent, and end up downloading one that is basically the same while the first is downloading, the new one will have it's file names shortened by removing the last character, and a "(1)" added, followed by two '.'(there should only be one) and then the extension.(BTW, What happens if it has two or more extensions? i.g. FILENAME.WITH.DOTS.txt.cpp.md5)
It definitely shouldn't reduce the name by one character, which will cause issues if the only difference in file names is the last character.
Note: I'm on Linux Debian Stable 9.x KDE

If it's going to rename the files it should say so or be more obvious that it's going to do that. and maybe offer some choices on how to deal with name conflicts.

-- related thoughts on issue --
I'm sure handling downloading two torrents covering the same file requires more thought than one that doesn't permit it. (pieces might not align, files might not actually be the same, etc...)
If sizes don't match then it's definitely not the same file. If there the same size then they might be the same file, but that's not guaranteed. If a set of downloaded pieces from one torrent's file completely cover a piece of the other one's torrent then the covered piece can be checked against the other torrent as included as a piece of the other torrents pieces. If it doesn't match, then the two files aren't the same, and a warning given or auto renamed. And then there's prioritization of these pieces doing it this way, doing this might make it better tor prioritize pieces that are next to each other, instead of rarest...

Related bug:
If no files are selected to run at start of torrent, no files will get checked if force check is run. It'd be nice if there was an option in the dialog box that was start paused and check files. (that way it can be seen if torrents are a close match before making comments to adding files, etc...)

On the last sentence, there is such possibility, use Stop button on the Load Transfer window, then hash check.