Why do you write "occurred" but "listened"?
Solution 1:
Occur has its stress on the final syllable (o-CUR) but listen (LIS-en) has its stress on the first syllable. If the verb ends in the pattern consonant-vowel-consonant, the final consonant is doubled in the case of verbs like refer and occur and begin where the stress is on the final syllable (referrer, referring, referred, occurrence, occurred, beginning), or one syllable words (big -> bigger, sin -> sinner) but not in the case of verbs like listen or broker (listened, listener, brokered, etc.), where the stress is non-final.