I just tried this with some of my own data and do not see the same behavior. Can you check the Description of your coded value domain and make sure the repeated text doesn't occur there?
Otherwise, can you send a zipped file gdb that contains the domain and the data so that we can take a look?
Save project. Delete current database connection, save and close Pro session. Open pro again with saved project and create new database connection. (Not confirmed workaround but worth trying if not tried).
I too noticed similar issue but then it went away, not sure which action cleaned it as I was doing several things.
The domains are fine as seen below:
I have this issue in other File Geodatabases using domains and subtypes, also using ArcGIS Pro 2.1.2.
This bug is also not system dependent (Win7 and Win10 systems).
When i open the GDB in ArcMap 10.5.1 the labels in the Contents menu are not duplicated.
Thanks for reporting this, Thomas. Yes, it appears that it happens when a subtype is applied to the feature class. I have logged the following bug:
BUG-000112731: With a subtype applied to a feature class, when the feature class is symbolized on a field with a domain applied, symbol labels are repeated.
This bug will now go to the development team to be addressed.
Thanks again for reporting this to us.