Welcome to Sig Talk!! Yeah, Mr Bumper is asking good questions, because the term 'double feed' can mean a bunch of different issues. More often than not, failures in new P series guns are extractor problems, where they'll change the extractor, & may be the spring too. But, it really doesn't matter - 'cause you need to call SIG.
They'll email you a Fedex 2 day shipping label. You'll need to put your gun & original mags into its plastic SIG case. I usually write a brief note to the gunsmith with the RMA number and what's wrong with the gun, the type of ammo I'd prefer he test with, etc. Unlike HK, SIG doesn't let us talk directly with their gunsmiths and this is just something I do, it's not really necessary.
Then pack the SIG case in a box and take it to your nearest Fedex location.
It's usually in Exeter in couple days, it'll be there ~5 work days and couple days return trip. Depending on weekends & holidays, it's usually a 2 or 3 week turnaround.
Cheers