I feel that it's now a bit too easy to get strange results. This is my test case:

So I have two separate objects (Cube, Cube.001) sharing the same data (CubeMesh). When I now choose Single User → Object Data Animation on Cube.001, the CubeMeshAction is duplicated to CubeMeshAction.001. So far so good. But, since the object data is shared, the new Action is assigned to CubeMesh, and both objects still have the same animation of their data:

Maybe it's enough to enfore "Object Data" when "Object Data Animation" is selected?

In D11683#303183, @Sybren A. Stüvel (sybren) wrote:

I feel that it's now a bit too easy to get strange results. This is my test case:

Yep, missed that, thx checking

Maybe it's enough to enfore "Object Data" when "Object Data Animation" is selected?

That makes sense to me, will update

Probably it is also good to check if the action has multiple users prior (otherwise all actions will be copied -- even the ones with only one user)