So far there hasn't been a case of someone knowing about a dupe and not reporting (AND revealing that fact).
But as far as I know, if you don't report, then all you're doing is not contributing to the site (like many other users who make an account and don't really do anything with it). You most definitely don't get banned.
Covering up for them though, like misleading the people to believe he's not a dupe, may warrant you a ban.