flag to list duplicate recipes

we have a setting to allow duplicate settings.

For personal projects, I always disallow and fail hard.
More and more I started to split generic recipes off from project-related recipes.

the generic non-project user-specific recipes import via this wrapper (until we have globbing supported :)
import? '~/.config/just/import.justfile'
but the result is obviously different for every user.

I want to allow duplicate recipes, but have more visibility what the duplicates are.
to avoid grepping and awking in custom scripts,
ideally I'd like to have a diagnostics flag to list them:

something like
```
just --list --warnings
just --list --verbose
just --list --duplicates
```

Start by tracing the existing --list behavior and how the imported ~/.config/just/import.justfile contributes recipes. Compare the proposed --warnings, --verbose, and --duplicates interfaces, then verify that duplicate recipes can remain allowed while the selected listing mode identifies them.

Written by the indexing model from the issue text.