Unless the gun has seen quite a lot of use, I'd suspect the issue is simply that you are not holding it as tight in the 'pocket' [onto/into your shoulder] as you used to. In other words, that you are loosely mounting the gun. It is perhaps the most common cause of having a gun double .. it is quite literally bouncing into your finger and firing twice in a very rapid succession and may well have zero mechanical issues involved.
If there is any staccato when it is doubling as opposed to a single 'big bang', that is the likely cause.
I'd suggest that you go back to the skeet field and make that determination first by mounting your gun quite solidly, if you are shooting pre-mounted. That type of 'doubling' is much less likely to occur when shooting low gun, but it can if the gun is not brought into the 'pocket' securely.
Once done to make that determination first, then I would do as Chuck & others suggest. If you are not comfortable doing it, then ask here for a competent gunsmith to do a 'strip & clean' in your area.
