If I have entered a contribution in duplicate, how can I delete one of them? I do not know what to try before asking my question.
Go into CiviCRM admin>Contributions>Find Contributions to find the relevant contribution(s). When the search is complete, on the very right hand side of the Find Contributions screen and in the relevant row, you will see the word "Delete". All you then need to do is click on "delete" and the contribution will be deleted.
Alternatively, if you are viewing a contact record, choose the "Contribution" tab where the contact's contributions will be listed. The "Delete" button is on the right hand side.