Sorry about the thread resurrection, but I feel it is still on-topic and best to keep the related items in the one thread. Found this thread via Google...
I have this happening too, and I've found a way to reproduce it (on Windows 7, alt.binz 0.39.4).
1. Open altbinz.exe, "Run as administrator"
2. Open altbinz.exe - either by importing an NZB or by double-clicking the EXE -- this creates a second instance
3. Open altbinz.exe - as per #2 -- this creates a third instance
Etc etc...
It works fine if the instances are opened as the same user e.g. if you try opening instances of alt.binz as non-administrator, it will only open up the one instance. Additionally, it works fine if a non-administrator opens alt.binz initially and then opens a second running as administrator.
I am not sure how you determine how many instances there are. Do you check for exactly one instance, in which case there could be two (running as different users) therefore tries to open a new one? Or a method that cannot identify multiple instances? Or something completely different...
Hope this helps... thanks.