That really sounds like a mag problem,in specific weak magazine springs.Double feeding occurs when the inertia from recoil strips the top round out the mag and then the gun tries to feed the next round as well,and failure to lock back the slide is also very symptomatic of a weak spring as well.I would try different mags(Mec Gar are great)and in the mags you have,replace the springs with Woolff +5% springs and see if that helps.One poster mentioned bad pinning of the mag,that happened to me with a Tanfoglio mag I bought a few years ago and the rivet was compressing a good part of the spring.