The words all ready, already (2 words/1 word) sound the same but have different meanings and spellings. Why do all ready, already (2 words/1 word) sound the same even though they are completely different words?
The answer is simple: all ready, already (2 words/1 word) are homophones of the English language.
ready in every particular; wholly equipped or prepared.
By this or a specified time: The children were already asleep when we got home.
So soon: Are you quitting already?
Informal Used as an intensive: Be quiet already. Enough already.
Definitions from the GNU version of the Collaborative International Dictionary of English, 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.