`cargo clippy --all-targets` issues diagnostics for library crate twice

Given a crate containing this file as src/lib.rs

pub fn demo() -> &'static str {
    return "demo";
}

and at least one file in tests that does use name_of_crate::demo, cargo clippy --all-targets will diagnose the needless_return twice in a row. If you use cargo --verbose it's clear that the clippy-driver program has been invoked twice (once with --test and once without) and each invocation is reporting the lint, but without that, it looks like clippy is repeating itself for no reason.

Reproduce with the shown src/lib.rs and a file under tests that imports demo, then run cargo clippy --all-targets and compare it with cargo --verbose. Trace the two clippy-driver invocations, one with --test and one without; done means the needless_return diagnostic is reported only once.