The words ducked, duct sound the same but have different meanings and spellings. Why do ducked, duct sound the same even though they are completely different words?
The answer is simple: ducked, duct are homophones of the English language.
Simple past tense and past participle of duck.
An often enclosed passage or channel for conveying a substance, especially a liquid or gas.
Anatomy A tubular bodily canal or passage, especially one for carrying a glandular secretion: a tear duct.
A tube or pipe for enclosing electrical cables or wires.
To channel through a duct: duct the moist air away.
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").