The words dammed, damned sound the same but have different meanings and spellings. Why do dammed, damned sound the same even though they are completely different words?
The answer is simple: dammed, damned are homophones of the English language.
Simple past tense and past participle of dam.
Condemned, especially to eternal punishment.
Informal Deserving condemnation; detestable: this damned weather.
Used as an intensive: a damned fool.
Used as an intensive: a damned poor excuse.
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").