The words done, dun sound the same but have different meanings and spellings. Why do done, dun sound the same even though they are completely different words?
The answer is simple: done, dun are homophones of the English language.
Past participle of do1.
Having been carried out or accomplished; finished: a done deed.
Socially acceptable: Spitting on the street is just not done in polite society.
To importune (a debtor) for payment: a dunning letter.
One that duns.
An importunate demand for payment.
An almost neutral brownish gray to dull grayish brown.
Definitions 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").