What are your criteria for duplicates? After all, in:
The quick brown fox jumped over a lazy dog. The flat cat ate a brown mouse.
the words 'The', 'a' and 'brown' are all repeated on the same line, but I'd hardly call any of them duplicates.

Also Word doesn't work in lines - it works in paragraphs. That's because what constitutes a line in a multi-line paragraph can vary according to what printer is attached, plus to simple things like changes to margins, etc. Word VBA also doesn't work in grammatical sentences.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]