Ensure package IDs are unique when cleaning.

get_many will panic if there are duplicate IDs.

src/cargo/ops/cargo_clean.rs

@@ -9,6 +9,7 @@ use crate::util::{Config, Progress, ProgressStyle};
 
 use anyhow::{bail, Context as _};
 use cargo_util::paths;
+use indexmap::IndexSet;
 use std::fs;
 use std::path::Path;
 
@@ -104,7 +105,7 @@ pub fn clean(ws: &Workspace<'_>, opts: &CleanOptions<'_>) -> CargoResult<()> {
     // Doc tests produce no output.
 
     // Get Packages for the specified specs.
-    let mut pkg_ids = Vec::new();
+    let mut pkg_ids = IndexSet::new();
     for spec_str in opts.spec.iter() {
         // Translate the spec to a Package.
         let spec = PackageIdSpec::parse(spec_str)?;