Comment 9 for bug 172390

running multiple instances is definitely a wanted feature - you can have logged in from different (geographically separated) terminals. that's not of interest to the user switching, but the existing bugs have to be fixed anyway. inherently exclusive tasks should provide locks (and, indeed, many do).
also, there might be session types that do not suffer from concurrency problems and i won't restrict them just because some [kde] apps happen to suck in this regard. so a warning is perfectly fine.
another option is of course redesigning the whole switching into a winxp/macosx style one - see bug #89023.