`-Zrustdoc-map` results in duplicate `--extern-html-root-url` for the same dependency

[Nemo157]

Problem

When building a crate with a big dependency tree the same dependency is included in the --extern-html-root-url flags many times, e.g. when building cargo-dl there are 72 duplicate entries for unicode-ident (presumably once for each position in the dependency tree):

> touch src/lib.rs && cargo rustdoc --lib -Zrustdoc-map -vv |& sed -e 's/ --/\n--/g' -e "s/ '--/\n'--/g" | rg -- --extern-html-root-url | sort | uniq -c | sort -n | tail
      6 --extern-html-root-url 'matches=https://docs.rs/matches/0.1.9/'
      8 --extern-html-root-url 'lazy_static=https://docs.rs/lazy_static/1.4.0/'
      8 --extern-html-root-url 'untrusted=https://docs.rs/untrusted/0.7.1/'
      9 --extern-html-root-url 'once_cell=https://docs.rs/once_cell/1.12.0/'
     10 --extern-html-root-url 'cfg_if=https://docs.rs/cfg-if/1.0.0/'
     14 --extern-html-root-url 'syn=https://docs.rs/syn/1.0.95/'
     16 --extern-html-root-url 'libc=https://docs.rs/libc/0.2.126/'
     29 --extern-html-root-url 'quote=https://docs.rs/quote/1.0.18/'
     58 --extern-html-root-url 'proc_macro2=https://docs.rs/proc-macro2/1.0.39/'
     72 --extern-html-root-url 'unicode_ident=https://docs.rs/unicode-ident/1.0.0/'

Steps

No response

Possible Solution(s)

No response

Notes

Likely caused by #13481, cc @GuillaumeGomez

Version

No response