This mod adds a way to bind multiple physical keys to the same action. Ill do an example here:
Let's say you want to bind the attack/break key to the left mouse button and the button 'n'.
Then you'd first open the config dupkeys-client.toml and set the value of kb1 to key.attack. Tos find out the action ids, you can take a look in options.txt.
Then from in-game in the Controls options, you can bind the Virtaul Key 1 to n and you'll be able to trigger the attack/break key with both left click and n.
The mod is a client side only mod and not required on the server.