Index terms can be duplicated #161
Description
Describe the bug
There's no check for duplicated index terms, so something can appear twice with no one complaining about it.
To Reproduce
Check out the generated search.js for duplicated terms. This is an outstanding issue in Raku/doc and could probably be done there, but even so, it's not bad to check it at this side.
Expected behavior
Warn or even fail when duplicated indices exist.