Thought I'd update this thread as a result of my current experience.

Have heard that the front trigger is made more "sensitive" as a result of the position and lever angles (etc etc) so that it feels to be about the same weight as the rear. This may be correct - my Merkel will double if I use the rear trigger first and with the standard 570gn load (a lighter 440gn load doesn't double though). But it does sound more like there is something wrong (haven't pulled apart yet) rather than a design error.

By the way I haven't actually experienced a true doubling but have noticed and confirmed when firing single loadings from alternate barrels.

I requested a parts break-down manual from Merkel so I could better understand what to expect before I pulled it apart, and so I didn't loosen the wrong sideplate screw first and have some spring loaded part dislodge before I could see where it's supposed to go/came from.

Anyway they sent a standard manual through and it mentions that the rifle is regulated for rapid firing (2nd barrel fired 6 to 10 seconds after the first) with the right barrel fired first. Probably regulates just as well left barrel first in most cases but they don't give guarantee of regulation this way (ie if it regulates fine left barrel first - fine, if it doesn't, well it wasn't supposed to anyway).