Add test and fix dedup

Author: estebank

src/librustc_resolve/resolve_imports.rs

@@ -1311,6 +1311,7 @@ impl<'a, 'b:'a> ImportResolver<'a, 'b> {
             is_redundant.present_items().all(|is_redundant| is_redundant)
         {
             let mut redundant_spans: Vec<_> = redundant_span.present_items().collect();
+            redundant_spans.sort();
             redundant_spans.dedup();
             self.session.buffer_lint_with_diagnostic(
                 UNUSED_IMPORTS,

src/test/ui/issues/issue-59896.rs

@@ -0,0 +1,10 @@
+#![deny(unused_imports)]
+
+struct S;
+
+fn main() {
+    use S;  //~ ERROR the item `S` is imported redundantly
+
+    let _s = S;
+}
+

src/test/ui/issues/issue-59896.stderr

@@ -0,0 +1,17 @@
+error: the item `S` is imported redundantly
+  --> $DIR/issue-59896.rs:6:9
+   |
+LL | struct S;
+   | --------- the item `S` is already defined here
+...
+LL |     use S;
+   |         ^
+   |
+note: lint level defined here
+  --> $DIR/issue-59896.rs:1:9
+   |
+LL | #![deny(unused_imports)]
+   |         ^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+