I've had maybe 3 or 4 instances of doubles firing on closing but never on opening. Does your gun by any chance have a particularly light trigger pull when it's set to fire bottom barrel first? Sounds like maybe a sear issue. When a sxs or OU doubles, it's a case of the first shot jarring loose the hammer from the other sear. May have been, for whatever reason, that your attempt to open the gun was enough of a jolt for a hammer that didn't quite recock correctly. Scary, but probably an easy fix.