Double letters at the end of words when adding -ing or -ed

When I was taught to spell many years ago, I learned that when you add a suffix like -ed or -ing at the end of certain words, you must repeat the last word of the letter. For example:

Travel -> travelling
Market -> marketting
Program -> programming
Focus -> focussing

This was important in phonetic spelling because if we didn't add the second consonant, the vowel would "say its own name"!

Now, I have noticed that when I type these words with double letters, they appear as spelling mistakes. I cross-referenced this with a Chinese student who studied ESL, and he said they learned the same rule about double letters at the end of words.

Does anyone know when/why this rule changed?