The words overdo, overdue sound the same but have different meanings and spellings. Why do overdo, overdue sound the same even though they are completely different words?
The answer is simple: overdo, overdue are homophones of the English language.
To do, use, or stress to excess; carry (something) too far.
To wear out the strength of; overtax.
Being unpaid when due.
Coming or arriving after the scheduled or expected time: <i>synonym</i>: <strong> tardy</strong>.
Expected or required but not yet having come about.
Delayed or with-held beyond the usual or assigned time: as, an <em>overdue</em> ship.
Definitions from The American Heritage® Dictionary of the English Language, 5th Edition., from The Century Dictionary. and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.