Binary 2s Compliment Applied Twice Gives Original - How?

Question

At the moment the maths behind binary 2s compliment seems like voodoo to me.

If I take 0011 (+3) and "flip and add 1", I get 1101 (-3)

If I apply the same process to 1101, ("flip and add 1"), I get back to the original positive value.

How is this so?