The reason im asking is that i originally had two amps in my car, but am changing to a single amp/ sub setup, so ive got a spare earth lead sitting in the boot on a different earth point to the other, my figuring would be that double earthing the amp would be beneficial anyone agree??

It won't hurt but surely only the stronger/cleaner route to earth would be used anyway?

(My understanding of GCSE Physics)

I can't see why, unless the earth leads are under-rated relative to the power they're being asked to handle.

The only benefit would be redundancy but that's about it.

As long as your lead is rated correctly and you have a good clean path to ground you should be fine.

well i mean its only sitting there the spare one, as its connected under the rear quarter panel, so i dont really fancy removing all that lot again to remove it, so might aswell make use of it

There is no harm in connecting it... but it will not do anything at all...

The only advantage you would see is if you're current earth was bad and the other one is better it would have a better connection... but if its working ok then it shouldn't need it but you may as well connect it so it isnt loose!

Well, if there's something to connect it to at the amp end without having to modify anything, including adding a piggyback connector to a terminal, it can't do any harm. It just won't do any good.