This is the correct spelling of the word in British/Australian/Indian/NZ etc., usage. Of course, some may find this spelling inappropriate here. From the internet:
benefited
Spelling: note the single t, which is because the syllable is unstressed.
© From the Hutchinson Encyclopaedia(UK). Helicon Publishing LTD 2007. All rights reserved
Thanks. Obviously, I was not aware.
This seems to be a fundamental difference between British and American spellings--sort of like the "-our" versus "-or" difference in so many words (neighbor, savior, flavor, etc.).
Compare, for instance, the British spelling of traveled, canceled, counseled, worshiped, labeled, leveled, and many other words. The typical American spelling is with a doubled consonant prior to the "-ed" (or "-ing") ending--although I have seen these words spelled the British way also, even in the US.