Warn/error if duplicate is allowed, but unnecessary

[sagudev]

Is your feature request related to a problem? Please describe.
We want to replace servo's python lints with cargo-deny, but this is only thing that is missing.

Describe the solution you'd like
Detect when a duplicate versions are allowed, but not present. This is useful to keep the list clean.

Describe alternatives you've considered
Do nothing.

Additional context
Issue in servo: servo/servo#34393
related code in servo: https://github.com/servo/servo/blob/d65a2e97970cbc3900c7f62e1c51776710623887/python/tidy/tidy.py#L375

[Jake-Shadle]

FWIW, though I fixed this bug, personally I always use exact version specifiers, as most of the time, skips are temporary when crate used transitively has done a non-patch release and it takes time for all dependencies to use the newer version, so when they all have the skip will no longer match the older version, and it will give an error if another version is somehow used in the graph during that time, unlike using just the crate name, which will match all versions.