Even though I don't have a set trigger, there is a difference between front and rear triggers. The front trigger is lighter than the rear (the manual says to pull the front one first for "regulation" purposes), and is articulated to hinge forward in case it hits your finger when the rear trigger is pulled.

I've played around with the rifle unloaded and the front trigger will "fire" that barrel if the trigger is pushed forward (as part of its articulation) and then allowed to snap back under spring pressure. I'm sure a heavier front trigger would fix the doubling issue, or maybe even locking the articulation in place? Not enough time at the moment to fiddle around with it and happy to abide by its rules of front trigger first.

By the way, I'm sure I've mentioned this earlier on this forum somewhere, there is a recoil operated interceptor on the left hand barrel / rear trigger sear lever (?) that blocks the sear lever from moving during recoil of the RH barrel - it's a pity that this wasn't fitted also to the other side.

The sear levers are the two bits extending up to the top right of photo (you can see most of the left hand lever and just the tip of the right hand lever):

The interceptor is the shiny hammer looking thing, and it's spring just below it, in front of it and secured by the screw behind it: