Coin duplication
Coin duplication, often shortened to coin duping, is a glitch that can cause the rock blocks in the Cascade Kingdom that normally drop a ring of eight coins to drop duplicate coin rings, in order to harvest up to 48 coins. The glitch involves hitting the blocks at a precise angle while using the T-Rex capture, and can be performed in the Cascade Kingdom overworld as well as the Dinosaur Nest room. When used in the Dinosaur Nest, it is the fastest known way of accumulating coins excluding Luigi's Balloon World.
Coin duplication is only possible on versions 1.0.1 and later, since it utilizes extra hitboxes of the T-Rex that were first implemented in 1.0.1.
Explanation
In versions 1.0.1 and onward, the T-Rex has six independent hitboxes that can all collide with a rock block at around the same time. This will cause the block to register up to six hits rather than only one, and drop a corresponding number of coin rings (any multiple of eight up to 48).
This is performed by walking into the corner of the block such that the T-Rex's body is above it with its legs straddling the sides of the block. Generally, it is easier to perform by walking into the block slowly rather than at full speed.