Found this at grammar.ccc.commnet.edu/grammar/spelling.
"Double the final consonant before adding an ending that begins with a vowel (i.e. "ed" or "ing")when the last syllable of the word is accented and that syllable ends in a single vowel followed by a single consonant." The first syllable of "visit" is accented and therefore according to the rule the "t" is not doubled.
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