Occur

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.