Raise error on duplicate editables

[charliermarsh]

If two editables resolve to the same package name, we silently drop one of them.

Try copying black_editable as duplicate_editable, then resolving:

-e ./scripts/editable-installs/black_editable
-e ./scripts/editable-installs/duplicate_editable

Which yields:

-e ./scripts/editable-installs/duplicate_editable

[charliermarsh]

(pip sync and pip install correctly fail.)