Words like boot, feet, wall, Morrison, etc
With the consonants, good luck. I have no more clue than you do.
With the vowels? Beet and bet sound very different. Took and tok sound different. Vowels get doubled to change their sound from short to long.
I assumed you meant when it becomes plural like gas and gasses (it is to prevent people assuming the e added is a silent e changing pronunciation of the word) of but looking at your examples…
Boot vs bot? The pronunciation would change. How would you show in English it is to be the long ‘o’ sound? When two different vowels are next to each other, the first vowel says it’s name and the other is silent (most of the time). So ‘boot’ and ‘boat’ and ‘bit’ are not pronounced the same. However, for your example ‘feet’, we do have ‘feat’ which denotes another meaning. Wall vs Wal? The two ‘L’s let you know it is a long ‘L’. How about ‘ball’ and ‘bal’? If I read the latter, I would assume it is pronounced like the first 3 letters of ‘balance’.
Not anything useful, if you were looking for a rule to memorize.
English spelling hasn’t changed since the invention of the printing press in the 1400s. There were “rules” (more like guidelines) used to spell Middle English words based on their pronunciation at the time, so those outdated rules have stuck even though the pronunciation of the words has changed. Then, loanwords into English tended to keep their original spelling, so the language of origin’s spelling rules can also determine where double letters are used (e.g. tortilla)