The above article [1] has been retracted as it was inadvertently published twice in the journal, Proceedings of the Edinburgh Mathematical Society. The first instance being Tan and Huang, 2021, doi: 10.1017/S0013091521000079 [2] which has already published into issue 64.2. The retracted article is published in this issue for full visibility.
The publishers apologise for this unfortunate duplication and would like to reiterate that this is not the result of any fault or error from the authors.