Refer

referring is one of the 117 mis-spellings listed as 'Common difficulties' in the section on 'Spelling' within 'Writing' in UEfAP.

As the stress in the verb 'to refer' falls on the final syllable, it follows the third of Hart's Rules on Consonant doubling: the final consonat ('/-r-') is doubled before '-ing'. '-ed', and '-ence'. Hence the correct spellings are

referring
referred

but as the stress is shifted to the first syllable of 'referee', it only has a singler '-r-':

referee