I have searched the forums, I'm not sure if this has been raised on sourceforge, I don't think so. I wouldn't usually leave a bug report like this so I'll apologise in advance but I think this may be the kind of thing that could go unnoticed for a long time.
It appears that when an alien on the battlescape leaves one soldiers LOS and moves *directly* into another LOS the alien is duplicated in both places. This has occured in the latest development windows build, I have yet to see it on Linux. Unfortunately, trying to reproduce this is a matter of random chance which is why I am putting it here. Sorry, no BT or debug info. I doubt my system specs or graphics hardware has anything to do with it, it rings of a logic bug but:
Intel 965 integrated
3GB RAM
Intel Mobile 4 Series
Windows Vista
I've only seen it once, on the small farmhouse map, I'll try to gather more info but as I said, it's random so others should look for it incidentally.