Lots of police officers carry backup pieces. I do at times.
There is a difference between carrying a backup gun, with paperwork out there showing you are the purchaser of the gun, versus a “drop gun”.
A “drop gun” is a gun which would be untraceable back to the cop, typically a stolen gun confiscated from some thug in the past. A cop planting a “drop gun” should be treated as having premeditated committing murder.