Multiple users booking exact same times and resources ?!?

We've been using PHPScheduleIt for over five years now, and am currently running a modded version of 1.2.10. Recently we've noticed an odd bug. We have many users trying to schedule on two particular resources on one schedule, and we're pretty sure it's turned into a clicking war. On three separate occasions, two users have booked the same time slots on the same resource. PHPScheduleIt will then show different times booked and move blackouts!

Example:
Fridays our equipment is available 9:30AM-12Noon, 1pm-4pm. 12noon-1pm and 4pm-10pm are blacked out.
User A and User B both log in and try to book time. User A books 1pm-3pm. User B books 1pm-4pm. Both click submit at the same moment (according to the scheduler: 9:00:25AM).
The scheduler shows the following: User A, 1pm-3pm, User B, 4pm-7pm, with the blackout moved.
If you click on the individual appointments, User A's says 1pm-3pm, User B's says 1pm-4pm, and the blackout says 4pm-10pm.

Any ideas what's going on? Things I can do to troubleshoot?

Thanks!