I have this list of choices:
Daily, weekly, monthly, yearly, once
The last one "once" is used to indicate thing that occurs only one time.
I wanted to keep up with pattern of the first four words. Is "once" the right word?
The xxxly pattern is semantically a short form for once every xxx. It means therefore that the event will by definition occur more than one time!
If something occurs only one time, and you want to keep the xxxly pattern, you would have to answer the question during which period of time this thing only occured one time, but whatever time period you choose, you would imply that the event will repeat itself.
So even if you say, as suggested in a comment, once in a lifetime, it means that the event would occur again in another lifetime (whenever that may be), so it is a repeating event.
I suggest you stick with once to indicate that an event occurs one time and that it does not repeat.
If it does repeat, but not very often, you could go with rarely or seldomly.
If you are filling a drop-down, once is correct. There is no ambiguity.
The operative word, that defines the context, which gives the issue meaning, is the implied label:
Which is followed by a list of words. There is no requirement to fulfil a pattern (unless you are laying tiles in the kitchen).
You could use NotRepeated or Not-repeated. I wouldn't call that disambiguation, but it does improve the meaning, the context.
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