Rollup merge of #154017 - GuillaumeGomez:duplicated-locs-scraped, r=Urgau

 Fix invalid add of duplicated call locations for the rustdoc scraped examples feature

Fixes rust-lang/rust#153837.

The bug was visible in the highlighting of the scrape examples code samples: the same part of the code was marked as "highlight" (by the scraped examples feature) twice, eventually leading to a wrong state of the HTML DOM status.

To fix the bug, I added a set to not re-add a location if it's already present.

r? @Urgau

Author: jhpratt