The general rule (called the 1-1-1 rule) is: 1) The root word must be one syllable long 2) The root word must have only one vowel 3) The root word must end in an unrepeated consonant 4) The suffix you are attaching must begin with a vowel If your word meets these conditions, double the last consonant. Having said this, I can think of two exceptions already: "referred" and "concurred" (both words violate rules 1 and 2). I'm sure others exist, but stick with the rules above and you should do well in almost all cases.
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