Is there any problem with CheckForDuplicateAppInstance running under Windows XP?
I have just tried running oneinst sample project and found I was able torun it mulple times without problems 
The system is WIndows XP PRO SP3 with full updates installed; the project is standard CVI2012 sample compiled and run multiple times. Ide is CVI2012 SP1 [ 12.0.1.127 ] the same as the run-time engine. CheckForDuplicateAppInstance returns no error and of course it returns no other instance running.
This is annoying since several of my customers are still using XP and I have this call in every application exactly to prevent more than one instance running...