2000:1::1 would expand to 2000:0001:0000:0000:0000:0000:0000:0001

2000::1::1 could be 2000:0000:0000:0000:0001:0000:0000:001, or 2000:00000000:0001:0000:0000:0000:001

There's ambiguity on where to fill in the five groups of 0000 in the second case.

The second address is invalid. You can only use :: once per address.

Edit: Whoops. Didn't read what the above post was in response to. My bad.