In that specific context, crates that have the same name within a Cargo crate graph but aren't the same exact crate - this is pretty much always about semver versions, when incompatible ones are required from different parts of the crate graph, you can end up with e.g. serde-1.0 and serde-0.9, and they get compiled with different -C metadata values, into which "1.0" and "0.9" were factored in.
