++once looks like an obscure way to pre-increment the parameter to twice
<once> looks much better, even though it might be slightly harder to type (ref: the vim discussion)

@ilAYAli angle brackets are onerous. Imagine if shell commands required every option to be wrapped in brackets.

++ looks like preincrement syntax, - looks like "negative", <> looks like HTML, ... syntax is always context-dependent, there's no avoiding that.

++ is for better or worse what Vim uses for :edit and :terminal. I would prefer hyphen, I don't understand why ++ is better than hyphen.

Merging this because it fixes 2 bugs.

@justinmk ++once looks ambiguous.
Regardless, both a hyphen prefix or angle brackets looks much better, and makes it clear that once is a parameter.

Any confusion about ++once is cleared up by the K command which takes the user to :help autocmd-once. There are zero Ex commands where ++ means "pre-increment".

Happy to rename it back to -once if someone convinces Bram to do so.

As it happens, both a lone - and double -- can be ranges (thus -once and --once is still theoretically valid commands), but on the other hand ++ is not a range, which would be a reason to prefer ++once .