The basic idea is that doubling the consonant is a signal to the reader to keep a short vowel short. For example:Could you please tell me if there is a rule to when you double a consonant before adding 'ing' to a word.
bet (short e) --> betting (still short e, not long e)
chat (short a) --> chatting (still short a, not long a)
slip (short i) --> slipping (still short i, not long i)
The problem is that if the consonant is not doubled, some people might be tempted to pronounce the vowels in beting, chating and sliping as long vowels. For an example that's a bit more illustrative of the fundamental problem, how about this:
tap (short a) --> tapping (still short a, but how would you pronounce taping?)
In words such as sleep, cheat, bleat, bloat, etc., it's not necessary to double the consonant because the vowel is long without the -ing and remains long with it. A word like peddle is another story because the basic word already has a short vowel and a double consonant to begin with.
Hope this helps.