Glock 20 double feed

[radiotom]

So this thing's been running like a champ for over 1000 rounds with my reloads and handloads. I replace the striker spring every 500 rounds and the recoil spring every 1000. I'm about 100 rounds into a new striker spring and 400 or so into this recoil spring. Yesterday while shooting I had a double feed. Not the extraction failure double feed, but one live round was 3/4 of the way into the chamber, and one from the magazine jammed right up into it. Tap rack didn't work. I had to clear the gun entirely and both rounds fired well. Ammo all felt the same...and I really don't suspect it was the culprit.

So...what causes a double feed? Was this just an extreme fluke? All of these magazines have been working great...

[The_Shadow]

It could be a fluke as simple as the top round slides forward as the slide cleared enough and then the slide dragging out a second round as it came forward.  That means the rounds could have been loose because the follower or rounds above the follower were stuck holding the spring down.
What comes to mind is the first round didn't fully slide up the breech face and into the chamber, thus not under the extractor.  Then the second round either tried to follow or during the clearing picked up the second round.