I have had this situation happen a few times where I found a device that was being supplied POE from an injector and that injector was plugged into a switch that is already providing POE. Obviously I remove the injector and it's done.
I have been having a problem with a users POE phone that has been having issues even after replacing the cables, phones and everything else. The phone would randomly stop working, or it would partially work and then it would work.
Anyways do POE injectors still work fine even though they may be plugged into a POE switch? Does the "double" power cause issues?
I just never heard of it causing issues so I am curious