occurred
Two Rs. The stress falls on the second syllable, so the consonant doubles.
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Wrong: occured
Right: occurred
Two Cs, two Rs.
The trick
Two Cs and two Rs. The rule: when a word ends in a stressed vowel-plus-consonant, that consonant doubles before -ed or -ing.
Compare refer → referred, prefer → preferred. But offer → offered, because the stress is on the first syllable there. Same rule, opposite result, which is why nobody trusts it.
Now go double the R.