What causes double feeds? (Glock)

[DeletePLS 178]

this is why you don't tinker with glock perfection 

[Howard 538]

Well I have not testing it in "battle" yet but it seems to be the recoil spring.  Since I did not trust it I shot a match in production (1st time in about 6 months) with my CZ, but brought the G35 and did a mag dump with the suspect ammo two rounds at a time and it worked flawlessly.  The only change was I put the original recoil spring in the gun.  So, perhaps the one I had been using was too light for this ammo.