One answer might be to set only one upload per person. So once they've finished, they can then choose another. Is it happening from another Limewire user or a different client?

This behaviour had been noted before here.

I suppose for example if you're sharing your downld folder, as well as your music library (eg: iTunes) then the same file might be listed twice in your shares. Have you searched your computer to see if you have more than one copy of the same file?