My boss offered me a weekend shift outside of my normal working hours, as overtime on a Sunday. I am contracted to work Mon-Fri with occasional weekends when required.The week prior to the Sunday I was ill on Monday and Tuesday and returned to work Wednesday. On returning to work on Wednesday I saw my boss had covered my overtime shift without checking whether I would be able to do it, is this allowed?
You state in a comment that overtime is not guaranteed in your job/locale so yes your boss can do this. While I can understand that it's unfortunate for you to miss out here it sounds as though your boss was just doing their job. The business needs the shift covered and with you out ill (and without knowledge of when you'd be returning) it makes sense to reassign the shift sooner rather than later and avoid getting to later on in the week and scrambling to get cover at short notice.
Yes your boss could have contacted you to see if you were still going to work the Sunday but you'd be surprised how many people would be annoyed by their boss ringing them up, while they were sick to ask if they were still going to work overtime.