Parker repro Doubling
 

What causes this? My dad had several repros. He's saying about 1/2 of his have this issue while the other 1/2 work as intended. Is there a certain part or failure point that causes this? What is the "right" way to solve it? Only happening in single triggers. I have a 20/16 repro with a single trigger and its been flawless.
Thanks,
Anthony

Gelled varnish inside the action. The varnish has been there since the ‘80’s and its time to clean and lube so it won’t double anymore.

Most of the time it is grease that has hardened with age. A good cleaning will usually fix the problem. There are directions on this site on how to remove the wood stock for cleaning. It's not too difficult if you have the right screw drivers. Don't try it if you don't have these as you will bugger the screws.

First solution, a good strip and clean. If that does not do it, the issue usually is that some of the cast parts are not cleaned up very well and need some smoothing out to make it all work correctly.