Add test for duplicate lang items

Author: Noratrieb

tests/ui/lang-items/duplicate.rs

@@ -0,0 +1,9 @@
+#![feature(lang_items)]
+
+#[lang = "sized"]
+trait Sized {}
+//~^ ERROR: duplicate lang item
+
+#[lang = "tuple_trait"]
+pub trait Tuple {}
+// no error

tests/ui/lang-items/duplicate.stderr

@@ -0,0 +1,13 @@
+error[E0152]: found duplicate lang item `sized`
+  --> $DIR/duplicate.rs:4:1
+   |
+LL | trait Sized {}
+   | ^^^^^^^^^^^^^^
+   |
+   = note: the lang item is first defined in crate `core` (which `std` depends on)
+   = note: first definition in `core` loaded from $BUILD_DIR/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-6cf4a3b928cf7834.rlib
+   = note: second definition in the local crate (`duplicate`)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0152`.