This is most obvious in regex.tex (maybe only here?)

For example, take a look at the table for syntax_option_type:
Immediately above the table, there is an index entry for the enum type, and for each enumerator that is defined in the table below. This produces the index entry with the page-number associated with the enum type (syntax_option_type). In the immediately following table, there is another index reference for each enumerator as it is defined. This produces a duplicate set of entries in the index, under a second entry for syntax_option_type that is not have a page-reference.

As far as I can tell, the index tags are character-for-character identical, so I presume it must be something in the index-generating logic that is treating these as distinct entries. This is a far as I can chase the issue without learning a whole lot more about LaTeX.