The words stoop, stoup, stupe sound the same but have different meanings and spellings. Why do stoop, stoup, stupe sound the same even though they are completely different words?
The answer is simple: stoop, stoup, stupe are homophones of the English language.
An act of self-abasement or condescension.
A descent, as of a bird of prey.
To bend forward and down from the waist or the middle of the back: had to stoop in order to fit into the cave.
To walk or stand, especially habitually, with the head and upper back bent forward.
Ecclesiastical A basin or font for holy water at the entrance of a church.
A drinking vessel, such as a cup or tankard.
Scots A bucket or pail.
A hot, wet, often medicated cloth used as a compress.
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").