I went ahead and paid myself back for a Door Dash credit that was already credited by the $60 Door Dash promotion and there was no problem. I wouldn't exactly call this a double dip since I could spend the points elsewhere but you can accomplish a quasi-double dip.
Grocery purchases that go against the travel credit do not appear in the portal as eligible for the pay yourself back option so no quasi-double dip is possible. I also notice that right after these post they disappear for a couple hours and the reappear along with the credit.