I've personally seen new rifles damaged with monos, and vintage rifles used with monos that weren't - yet. Holland & Holland has had NEW (brand new, with barrels of top quality modern steel) double rifles ruined before they were even finished (being regulated in the white) with monos. The "new vs old" argument regarding monos in double rifles is pure myth. It's a matter of wall thickness and nothing else. A clunk with .125" min wall will probably handle quite a few before damage occurs, but it will eventually.

The most insidious part of the problem is that so few even know what it looks like. I know a guy, a VERY sharp and experienced double rifle guy, who bought a very expensive best DR that had serious rifling imprint on the outside of the barrels. He didn't have clue until his gunmaker saw it.