The words junkie, junky sound the same but have different meanings and spellings. Why do junkie, junky sound the same even though they are completely different words?
The answer is simple: junkie, junky are homophones of the English language.
Slang A narcotics addict, especially one using heroin.
Slang One who has an insatiable interest or devotion: a sports junkie.
Of or related to junk; worthy of being discarded.
Meaningless, fatuous, or unbelievable: a junky novel.
Variant of junkie.
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.