A great way to frame DRY that I heard from hackernews: "DRY things that are supposed to have the same behavior, not things that happen to have the same behavior"
This is a really good way to put this. The "just because the do they same thing right now doesn't mean they _do the same thing_" concept is hard to convey!
I enjoy Sandi Metz' point there as well: Code just looking the same is not enough to call it duplication. Once you have to change two places looking the same to add a new feature or to fix a bug, then you have duplication and should centralize it.
I usually wait till 3; 2 is about the _very general_ point at which changing multiple places is about the same work as changing it to be centralized. 3 is almost always a better place to make that leap.
