When we have one vowel and one consonant and we want to add 'ing', we usually double the last consonant.
Why don't we add an extra 'x' to the word 'fix'?
We don't double 'w' and 'y' maybe because they are semi-vowels but what about 'x'?
Orthography – Why ‘Fixing’ is Written Instead of ‘Fixxing’
doubled-consonantsorthographysuffixes
Best Answer
The letter x is never doubled as in tax: taxing, taxed. Probably because x is not found double in other terms.
(howtospell.co.uk)
From Dictionary Of The British English Spelling System | Greg Brooks