How can I make a contact the case manager of several cases with the same client contact?
So, I know it's against best practices, but after digging into the code a bit, I've located a (hacky) solution to my problem. When adding a Case Manager via a case view, CRM_Contact_BAO_Relationship::legacyCreateMultiple is executed.

At line 194 there's a check for duplicate relationships. If I comment out the "continue" at line 203 within the dupe check, I can successfully assign the case manager to the case.

I would much rather not have to do a core edit, so I await a better solution.

Edit: I had run into a problem elsewhere that ended up being caused by my hack. I do not recommend implementing my hack as I'm not sure of the implications.