Handle rustc_on_unimplemented in duplicated_attributes

Author: Noratrieb

clippy_lints/src/attrs/duplicated_attributes.rs

@@ -37,9 +37,10 @@ fn check_duplicated_attr(
     }
     let Some(ident) = attr.ident() else { return };
     let name = ident.name;
-    if name == sym::doc || name == sym::cfg_attr {
+    if name == sym::doc || name == sym::cfg_attr || name == sym::rustc_on_unimplemented {
         // FIXME: Would be nice to handle `cfg_attr` as well. Only problem is to check that cfg
         // conditions are the same.
+        // `#[rustc_on_unimplemented]` contains duplicated subattributes, that's expected.
         return;
     }
     if let Some(direct_parent) = parent.last()

tests/ui/duplicated_attributes.rs

@@ -15,4 +15,17 @@ fn foo() {}
 #[cfg(unix)] //~ ERROR: duplicated attribute
 fn bar() {}
 
+// No warning:
+#[rustc_on_unimplemented(
+    on(
+        _Self = "&str",
+        label = "`a"
+    ),
+    on(
+        _Self = "alloc::string::String",
+        label = "a"
+    ),
+)]
+trait Abc {}
+
 fn main() {}