Factory or reloads? I hope you meant PPK!

Regardless if factory or handloads, blowback guns in particular are subject to incomplete chambering if the bullets are slightly over sized, or the cartridge is over length. If the slide is held even a few thousandths of an inch from fully closing the hammer can strike the slide without touching the firing pin. This can move the slide enough that the firing pin will be struck properly the second time.

Pull the slide off the gun and use the chamber as a case gauge. If the ammunition drops cleanly into the chamber, and then falls out freely when the barrel is turned muzzle up everything should be fine. If the ammunition sticks either way you have found the problem.

Are you loading the magazine and letting the gun function normally, or single loading and letting the slide down gently instead of letting it close with full force of the recoil spring pushing it? If so, and your post sounds like this is what you are doing, let the gun do its' job normally and let's see what happens.

If you still have a problem we would need a bit more information to even venture a guess.

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Gunsmithing since 1961