When usually is used as an adverb of frequency, it suggests a greater frequency than often to me. However, usually can mean in a way that is normal, and it need not imply 'often':
I go to England about four times a year. I usually drive, but I fly if I have a lot of work to catch up on.
Incidentally, in that example, 'often' could be used instead of usually, with the same meaning.
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