Incorrect spelling, explanation: this is a mistaken form because the only case we double the last consonant in this word is when adding some suffixes (for example occurring). The base form, however, remains with one r – occur. Therefore occur is correct, while occurr is a misspelling.

Correct spelling, explanation: there is no need to double the last r in occur in its general form – lexical nor pronunciation-connected. Of course, according to the rule of doubling the last consonant in the case of consonant-vowel-consonant ending combination, this last r will double when we add any suffix to it (for example occurred). However, in the base form, it remains with one r. Therefore occur is correct and occurr incorrect.

Definition of occur:
verb, to appear, to take place
If clouds occur in the sky, it will probably rain soon.
Behaviour like this shouldn’t occur in the classroom.

Incorrect spelling

Incorrect spelling

Incorrect spelling