I have this problem, too, but I'm quite sure it has nothing to do with that alt.binz would redownload the files. It's a different problem. In fact everytime these "something.blaxxxx" files appear, the decoding has f*cked up. I can sometimes see it happening:
Normally the decoding progress bar runs through and afterwards the just downloadeded file gets deleted from the download queue. However, sometimes the decoding stops at e.g. 30%, then starts again from the beginning. This time it goes to 40%, then it starts again. Everytime the decoding restarts, a new "xxxx" file is created.
Next clue: Everytime such an "xxxx" file is created, the previous "xxx" files including the "correct" file without any "xxx"s cannot be renamed, anymore. This can be fixed only by restarting alt.binz. In such a situation repairing and unraring also fails, because alt.binz itself can't open such files for writing, anymore, either.
Next clue: Normally there's always only one "decoding" progress bar at a time. But now and then I'm seeing two bars. One bar is stuck, the other one is running. I think that's not as it was intended.
I think there's some kind of timing problem or some kind of thread synchronization issue or some kind of file handle leak, which is causing all these problems. It has nothing to do with the anti virus software. It happens with AV disabled, too. However, the problems seem to increase if I run a full screen DirectX game while alt.binz is downloading. I may be wrong with that, maybe the game doesn't hurt alt.binz, it's just my impression.
This is about the only thing the troubles me when using alt.binz. Apart from that it's great!