reftex-index-phrases-skip-indexed-matches

reftex-index-phrases-skip-indexed-matches is a variable defined in `reftex-vars.el'.
Its value is

nil

Documentation:
Non-nil means, skip matches which appear to be indexed already.
When doing global indexing from the phrases buffer, searches for some
phrases may match at places where that phrase was already indexed. In
particular when indexing an already processed document again, this
will even be the norm. When this variable is non-nil, RefTeX checks if
the match is inside an index macro argument, or if an index macro is directly
before or after the phrase. If that is the case, that match will
be ignored.

You can customize this variable.