The problem with what you're asking is that there is no reliable way of determining what a duplicate word or phrase is. Even in your example, 'many' is not a literal duplicate of 'Many'. Also, what happens if there are other words between your duplicates, for example:
Many years ago I did many years ago a nice job.
For all anyone except the author knows, the correct expression after duplicate removal might be:
Many years ago I did many a nice job.
or:
Many years ago I did a nice job.
Both are grammatically and linguistically valid.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]