I've seen several Merkels double discharge when the left barrel was fired first. At our DRSS hog hunts we shoot a lot of different DRs, and quite a few Merkels show up.

One guy has a new 141 in 9.3. The first time I watched him shoot it, he double discharged it three times in a row, using the rear trigger first. At the next hunt six months later, he double discharged it twice again, still using the rear trigger first. I then fired two pair with it in the normal R/L order with no problems.

A good friend had a Model 140 in .500 NE. It was developing a hairline crack in the stock head on the left side, but had otherwise been fine. At another of our hunts, a visiting Zim PH (a young, big guy) wanted to try it. Worried about a double discharge, he pulled the rear trigger first. When both barrels went off, the recoil drove the front trigger into his trigger finger so hard that the trigger was bent foward and out to the right and ended up sticking out of the trigger guard. His finger was cut to the bone. The owner had it fixed and sold it.

I remember another Merkel that did this, but can't remember the specifics now. Merkels are the only DRs I've seen double discharge when the left barrel is fired first. From the comments posted above, it may be more of a recurring problem than I had thought.