Occur
From Hull AWE
The recommended forms of the verb 'to occur' are:
- past forms occurred;
- present tense 3rd person singular (and plural of the noun) occurs;
- present participle occurring.
- The related noun is occurrence.
This is the advice given by Hart's Rules: as the stress in the verb 'to recur' falls on the final syllable, it follows the third of Hart's Rules on Consonant doubling: the final consonant ('-r-') is doubled before '-ing'. '-ed', and '-ence'.
- You may also want to see AWE's article on Consonant doubling.
Occurred and occurrence are among the 117 mis-spellings
listed as 'Common difficulties' in the section on 'Spelling' within 'Writing' in UEfAP.