The past tense of to occur is occurred (not occured), but the past tense of to listen is listened (not listenned).
Why? What is the general rule that is applied to make the past tense of a verb?
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.
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I guess it's to retain pronunciation. If you don't double the ending consonant, it will be ok-cured (occured) instead of occur-ed (occurred), buy-ger (biger) instead of big-er (bigger), bay-red (bared) instead of bar-ed (barred), etc.
Seems intuitive and natural to me, though I can't confirm. If I'm right, the general rule would be not to double unless pronunciation changes.