I've configed certain blocks to drop different items from what they normally drop, but in-game when players break these blocks, it drops the items they normally dropped and the item I defined. I have set drops to override and default drops to clean but it doesn't seem to be working.

My config: http://pastebin.com/xiCN4SD7
Server version: http://prntscr.com/r7a4u

I'll try and take a closer look this weekend. Thanks for posting your config.

No problem, let me know if you need anything else.