Suzanne is right: the speller identifies repetitions automatically and there
is no special function or option you need to activate to do so. Don't
hesitate to tell us if you have legitimate duplications which the speller
does not find. If you type something like "He read the the book", the second
occurrence of "the" should be red-squiggled. If you right-click on it, you
can choose to "Delete Repeated Word".
Best,
Thierry
Thierry Fontenelle [MSFT]
Microsoft Natural Language Group