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.
do something to an excessive degree
To do too much of something.
To cook too much.
To surpass; to excel.
past due; not paid at the scheduled time
<xref>Late</xref>; especially, past a <xref>deadline</xref> or too late to <xref>fulfill</xref> a <xref>need</xref>.
Due and more than due; delayed beyond the proper time of arrival, payment, return, etc.
Unpaid at the time assigned or agreed on: as, an <em>overdue</em> bill.
Definitions from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved., from Wiktionary, Creative Commons Attribution/Share-Alike License., from the GNU version of the Collaborative International Dictionary of English., 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.