Officially, Durge isn't Mando. Never was.
Durge was trained by Jaing, and was known as Jaing's apprentice. The Attack of the Clones Visual Guide says that Boba was being trained as Jango's apprentice in the Mandalorian ways, which eventually got expanded on by Karen Traviss to show that Mando kids are apprenticed to their parents for training up until they're 13. So there's that. And throughout the Dark Horse Clone Wars comics, Durge has a pretty familiar-looking skull painted on his armor
. Yes, the EU can't get it's collective crap together and say 100% one way or the other whether the Mando skull is from the mythosaur or Jaing, but that's something to think about.
It could be that Durge was just interested in learning how to fight from somebody who was a master fighter, and ends up wearing the symbol out of respect for Jaing. Which seems to be the current canon explanation. But I think it could
be argued that Durge was training to be a Mando when that got derailed by Jaing getting killed by a bunch of bad Mandos...he strikes back at the bad Mandalore...gets captured, tortured, and buried alive by other Mandos in response...and ends up the absolutely insane Mando-hating guy we all know and love.
Just putting that out there.