You should bring this up in the smo category extensions forum

This is a serious question with significant importance also for 100%, All Missions, Any 999, and Max% (Skelux%). If you hit a rock as the T-Rex just right, the rock drops its coin or coin ring up to 6 times, which is known as Coin Ring Duplication. By carefully executing a setup similar to Stravos96', this glitch can be triggered somewhat consistently, resulting in a higher average coin collection rate than Bowser's Kingdom. This means that, with enough skills and effort (and soberness, sorry Oki), it is possible to grind any amount of coins faster than with the old method. For reference, see Stravos' speedrun to 9999 coins where he grinded from 269 to 9999 coins in 46:28 minutes: