Don't treat host/target duplicates as duplicates

src/cargo/ops/tree/graph.rs

@@ -233,9 +233,33 @@ impl<'a> Graph<'a> {
 
         let mut dupes: Vec<(&Node, usize)> = packages
             .into_iter()
-            .filter(|(_name, indexes)| indexes.len() > 1)
+            .filter(|(_name, indexes)| {
+                indexes
+                    .into_iter()
+                    .map(|(node, _)| {
+                        match node {
+                            Node::Package {
+                                package_id,
+                                features,
+                                ..
+                            } => {
+                                // Do not treat duplicates on the host or target as duplicates.
+                                Node::Package {
+                                    package_id: package_id.clone(),
+                                    features: features.clone(),
+                                    kind: CompileKind::Host,
+                                }
+                            }
+                            _ => unreachable!(),
+                        }
+                    })
+                    .collect::<HashSet<_>>()
+                    .len()
+                    > 1
+            })
             .flat_map(|(_name, indexes)| indexes)
             .collect();
+
         // For consistent output.
         dupes.sort_unstable();
         dupes.into_iter().map(|(_node, i)| i).collect()

tests/testsuite/tree.rs

@@ -984,6 +984,51 @@ cat v2.0.0
         .run();
 }
 
+#[cargo_test]
+fn duplicates_with_target() {
+    // --target flag
+    if cross_compile::disabled() {
+        return;
+    }
+    Package::new("a", "1.0.0").publish();
+    Package::new("dog", "1.0.0").publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.1.0"
+
+            [dependencies]
+            a = "1.0"
+            dog = "1.0"
+
+            [build-dependencies]
+            a = "1.0"
+            dog = "1.0"
+
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file("build.rs", "fn main() {}")
+        .build();
+    p.cargo("tree -d").with_stdout("").run();
+
+    p.cargo("tree -d --target")
+        .arg(alternate())
+        .with_stdout("")
+        .run();
+
+    p.cargo("tree -d --target")
+        .arg(rustc_host())
+        .with_stdout("")
+        .run();
+
+    p.cargo("tree -d --target=all").with_stdout("").run();
+}
+
 #[cargo_test]
 fn charset() {
     let p = make_simple_proj();