--- /dev/null
+#![deny(unused_attributes)]
+
+#[doc(alias = "A")]
+#[doc(alias = "A")] //~ ERROR
+#[doc(alias = "B")]
+#[doc(alias("B"))] //~ ERROR
+pub struct Foo;
+
+fn main() {}
--- /dev/null
+error: doc alias is duplicated
+ --> $DIR/duplicate_doc_alias.rs:4:7
+ |
+LL | #[doc(alias = "A")]
+ | ----------- first defined here
+LL | #[doc(alias = "A")]
+ | ^^^^^^^^^^^
+ |
+note: the lint level is defined here
+ --> $DIR/duplicate_doc_alias.rs:1:9
+ |
+LL | #![deny(unused_attributes)]
+ | ^^^^^^^^^^^^^^^^^
+
+error: doc alias is duplicated
+ --> $DIR/duplicate_doc_alias.rs:6:13
+ |
+LL | #[doc(alias = "B")]
+ | ----------- first defined here
+LL | #[doc(alias("B"))]
+ | ^^^
+
+error: aborting due to 2 previous errors
+