Sometimes it is hard to know whether you need one consonant or two in the middle of a word when it has two syllables.
For example, is it ketle or kettle?
The first 'e' is a short vowel (like the 'e' in met) so you need to double the 't'. The correct spelling is kettle.
If the vowel is short (like the 'a' in cat, the 'e' in met, the 'i' in hit, the 'o' in pot and the 'u' in cut) then you double the consonant.
The 'a' is short, so the correct spelling is rabbit, not rabit.