Ori/Key Duplication

Key duplication is a trick that can be performed on any keystone door, provided that you can die while in range of the door. If your last save is outside of the scene containing the keys, then any keys inside the door will be returned to your inventory on death, and they will also respawn when the scene reloads. Here is a video of a common key dupe. Note that for safety, you could land next to the door first then edge off the platform, but the idea remains the same.