Double feeds are one of the worst and quickest ways to bend an extractor spring. The extra round pushes against the one that it in the bolt head and forces it against the extractor with significant force. You're probably getting that due to a lack of ejection. Make sure the ejector spring (at the rear of the ejector and in the top rear part of the trigger pack) is mounted correctly and keeping the ejector lever up when there's not pressure on the rear. And as stated before, make sure the ejector is alligned correctly. Look at the top rear part of the ejector lever to make sure there are no little nicks in it from the bolt carrier, or any signs of binding.
Allignment on the ejector can be checked by holding the gun upside down, with the magazine removed, and slowly cycling the bolt assy while looking into the magwell and checking to see that the ejector tip always goes easily into the bolt head slot as the assembly moves rearward. Sometimes, after lots of FTEs, binding can cause the ejector tip to get out of allignment. So this test will check for that.