The words ha ha, ha-ha sound the same but have different meanings and spellings. Why do ha ha, ha-ha sound the same even though they are completely different words?
The answer is simple: ha ha, ha-ha are homophones of the English language.
Expression of laughter.
A sound made in imitation of laughter.
Slang An instance of amusement. Often used in the plural: drove past the old school just for ha-has.
Used to express amusement or scorn.
See sunk fence.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").