[ ]{2,}
SPACE (2 or more)
You could also check that before and after those spaces words follow. (not other whitespace like tabs or new lines)
w[ ]{2,}w
the same, but you can also pick (capture) only the spaces for tasks like replacement
w([ ]{2,})w
or see that before and after spaces there is anything, not only word characters (except whitespace)
[^s]([ ]{2,})[^s]
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