Do not double last letter if it's a w, x, y - tax - taxed, stay - staying, towed...
Do not double if two vowels - heat - heating, sleeping, sleeper...
Do not double if two consonants - sharp- sharpen, fasting, turned...

https://howtospell.co.uk/suffixes-and-spelling-rules

https://howtospell.co.uk/doubling-up-rule

I hope this helps.

By the way, that was a very good question, you are very perceptive indeed!