I’ve never incurred on this problem on my Linux box so I was very surprised.
Booting under Windows I’ve found this different behavior, very strange because the GIMP documentation explains how to obtain the opposite effect using command line switches, i.e. running always new instances.
IMHO the reuse technique under Windows is bugged so I found a workaround using the not so famous gimp-win-remote executable program present on bin directory.
gimp-win-remote checks (correctly) if GIMP is already running and reuse it.
I’m curious to know if Mac OS/X complains to unix world, if you have a Mac and would satisfy my curiosity I will update the ViewSourceWith FAQ page 😉