How come for 100's of years, this "doubling" hasn't been a problem yet all of a sudden, "Merkel" have a problem product ?

Sounds to me like people have become so used to Single trigger shotguns they have lost the ability to use double triggers and / or they need to get their technique right.

I have 2 Merkel's, neither is a problem, both shoot well.
I also have multiple other double rifle's, and only one ever doubled and that turned out to be worn mechanism which was fixed.

And as for blowing up, well, what can you, if it had it would have been all over every internet forum.

The whole things sounds like more internet bullshit
rehashed.