Before sending the gun out, change the selector and fire the other barrel first. If it does not double, you have a problem. If it does, let some one else shoot it. If it does not double on them, you are fan firing. Not holding the gun tightly enough can cause this. If it doubles with other shooters, then you have a problem.
Doug Braker, AKA Gun Dr can probably help. I know of several folks, but Doug frequents this site and has a good reputation among TS.COM folks. These guns are long out of production. This was one of Browning's dumber moves.
I saw his post on the "Long Browning Firing Pins". PM him and see if he can help.