Disallow duplicate `implements`

[pcaversaccio]

This currently compiles:

from ethereum.ercs import IERC20

implements: IERC20
implements: IERC20  #  Duplicate `implements`.

balanceOf: public(HashMap[address, uint256])
allowance: public(HashMap[address, HashMap[address, uint256]])
totalSupply: public(uint256)


@external
def transfer(to: address, amount: uint256) -> bool:
    return True


@external
def approve(spender: address, amount: uint256) -> bool:
    return True


@external
def transferFrom(owner: address, to: address, amount: uint256) -> bool:
    return True

I think it would be sane to disallow duplicate implements of the same interface. See also #4772 (comment).