While conducting department defensive tactics/firearms training involving clearing weapon manipulations with the off-hand only, one of our officers found an extremely simple way to pull a magazine from a pistol with a double-feed using only the support hand. This is usually fairly tricky as a double feed puts pressure on the magazine, making it difficult to pull it from the pistol using only one hand.
The officer placed the pistol on the ground with the magazine release button facing up and grasped the slide of the weapon with her off hand, pressing the magazine release button with her thumb. She then placed her knee on the pistol grip of the pistol, putting pressure on the end of the magazine. She was then able to pull the weapon away from the magazine, which was left under her knee.
This method is extremely simple and is also very effective.