Steps to reproduce

To reproduce:

$ cd $(mktemp -d)
$ spack -c config:install_tree:root:./here install py-setuptools
$ spack -c config:install_tree:root:./here spec py-joblib ^/hash-of-just-installed-python

This somehow causes python to appear twice with a different Spec instance (different object id) in the same link-run subdag, once through the path:

py-joblib -[br]-> py-setuptools -[br]-> python

and once directly

py-joblib -[br]-> python

this confuses Spack internals that rely on id instead of dag_hash as a key.

It would be better though to ensure that id(s) == id(t) for any s and t in the link/run subdag where s.dag_hash() == t.dag_hash(), instead of duplicating the instance.

This causes problems, such as the assertion here, which doesn't traverse with dag_hash as a key:

• Spack: 0.21.0.dev0 (0e27f05)
• Python: 3.11.4
• Platform: linux-ubuntu23.04-zen2
• Concretizer: clingo

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output