I'm still trying to get better details, but I've seen multiple cases now of builds that don't have concurrent builds enabled launching two builds at once - this is only showing up in 1.363, after upgrading from 1.346. I'm wondering if it might be related to the changes for http://hudson.361315.n4.nabble.com/Patch-to-fix-concurrent-build-problem-td2229136.html that went into 1.363. I'll update with more info if/when I can reproduce further.