Hiya, thanks for the time you’re putting in to improving Heysham port. Unfortunately this water area was not a duplicate: it marked the bounds of the harbour area, and is important. It was incorrectly marked as a dock rather than a harbour, which I’ve fixed. See https://wiki.openstreetmap.org/wiki/Harbour

I’ve re-added and improved it in the following changesets. Please let me know if anything in them looks wrong. Thanks.

https://www.openstreetmap.org/changeset/158000303

https://www.openstreetmap.org/changeset/158000779

https://www.openstreetmap.org/changeset/158000880