Hmm I've definitely run into this QC bug before but not as often as you seem to be. Do you also navigate away from the test/resource in QC before the other person accesses?
Object Lock error in QC - HP Enterprise Business Community
"Items get locked in this way for various reasons. The most common reasons are: - User session timed out while the item was open, - User failed to logoff the project correctly (closed the browser using the X), - User's browser session was interrupted due to clicking a link in an email (for example), - Browser encountered an error and had to shut down."