You put them into a "common" or "helpers" file or module and you do code reviews in the team. Even if you're not completely up to date on what's already there

You put them into a "common" or "helpers" file or module and you do code reviews in the team. Even if you're not completely up to date on what's already there you'll probably bump into it when trying to write your helper in the same place. It's not a silver bullet, of course, but don't let perfect be the enemy of the good.