XCM: Process PayFees only once (#7641)
The `PayFees` instruction should only ever be used once. If it's used more than once, it's just a noop. --------- Co-authored-by:cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com> Co-authored-by:
Adrian Catangiu <adrian@parity.io>