Add a test

src/test/ui/deduplicate-diagnostics.deduplicate.stderr

+error: cannot find derive macro `Unresolved` in this scope
+  --> $DIR/deduplicate-diagnostics.rs:4:10
+   |
+LL | #[derive(Unresolved)]
+   |          ^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/deduplicate-diagnostics.duplicate.stderr

+error: cannot find derive macro `Unresolved` in this scope
+  --> $DIR/deduplicate-diagnostics.rs:4:10
+   |
+LL | #[derive(Unresolved)]
+   |          ^^^^^^^^^^
+
+error: cannot find derive macro `Unresolved` in this scope
+  --> $DIR/deduplicate-diagnostics.rs:4:10
+   |
+LL | #[derive(Unresolved)]
+   |          ^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

src/test/ui/deduplicate-diagnostics.rs

+// revisions: duplicate deduplicate
+//[duplicate] compile-flags: -Z deduplicate-diagnostics=no
+
+#[derive(Unresolved)] //~ ERROR cannot find derive macro `Unresolved` in this scope
+                      //[duplicate]~| ERROR cannot find derive macro `Unresolved` in this scope
+struct S;
+
+fn main() {}