BandHelper doesn't support using the same file for multiple songs. If you want to do that, you can add a separate copy of the file to each song, or you can place the same song into your set list multiple times.
However, I have to revoke my earlier statement that you should not select a file when using the Copy to Devices Manually option. If you don't select a file, BandHelper won't know the file format of the file you're using and won't be able to connect to it. I just updated the website to require a file selection, although the file won't actually be uploaded so you won't hit the upload size limit. This same change will be in the next app versions.