Problem

There are situations in Rust when dependencies used by multiple crates should have the same semver-compatible versions (when they export traits or types shared across crates).

The problem is that cargo update or other lock file-rebuilding commands can change a project with no (unwanted) duplicates into a project with duplicates.

Duplicate dependencies are sometimes necessary or even desirable, but they can also happen by accident without users being aware of the problem.

Currently Cargo reports "Added dep-name 1.x", but that log message doesn't get any special treatment when the dependency is another version of a dependency that has remained on previous version.

Duplicate crates can cause compilation errors that rustc can't present nicely, because rustc doesn't know crate versions and doesn't know what caused them to be added to the project.

Proposed Solution

Cargo printing a warning along the lines of "Added duplicate dep-name 1.x, because other-dep requires ^1. Another dep-name version is 0.y, because different-dep requires 0.*" (or even print the whole "path" of dependency requirements up to the root)

This could help users spot the problem earlier, with more precise root cause information. Currently investigation requires running cargo tree -d and cargo tree -i, which uses may not know about, and looking up this info manually is more laborious than having it presented automatically when dupes happen.

Even when dupes don't cause compilation errors, they can slow down builds and bloat executables, so it is in users interest to avoid having duplicate dependencies when it's not necessary.

### Tasks
- [ ] Add a draft title or issue reference here