It's redundant, yes, but 'only' serves a purpose there. 'once' can also mean, on a specific occasion/at one time in the past and that could in some contexts lead to ambiguity. For example,
I used each word once. (two meanings; one word, once; on one occasion)
Once I used each word. (one meaning; on one occasion)
When it comes to meaning, clarity trumps redundancy.
As for your example, it doesn't admit ambiguity, so 'only' isn't required.
All the best.
Interested in Language