Even with merge policies, if someone doesn't understand what they're doing or how it works, it won't help.

Last project I worked on, we asked our developers to rebase on the parent branch and squash on their pull requests. But, they often encountered conflicts because they never updated their branch. They would pull the parent branch into their feature breach, or do a regular merge, or merge different other branches together and you'd end up with commit duplication everywhere. They didn't have any discipline.