diff --git a/crates/ailang-surface/src/parse.rs b/crates/ailang-surface/src/parse.rs
index 2e9af37..ed68f69 100644
--- a/crates/ailang-surface/src/parse.rs
+++ b/crates/ailang-surface/src/parse.rs
@@ -373,8 +373,17 @@ impl<'a> Parser<'a> {
         loop {
             match self.peek_head_ident() {
                 Some("doc") => {
-                    let s = self.parse_doc()?;
-                    doc = Some(s);
+                    if doc.is_some() {
+                        let pos = self.peek().map(|t| t.span.start).unwrap_or(0);
+                        return Err(ParseError::Production {
+                            production: "data-def",
+                            message: format!(
+                                "data `{name}` has duplicate `(doc ...)` clause"
+                            ),
+                            pos,
+                        });
+                    }
+                    doc = Some(self.parse_doc()?);
                 }
                 Some("ctor") => {
                     ctors.push(self.parse_ctor()?);
@@ -2474,4 +2483,23 @@ mod tests {
             "expected duplicate-body diagnostic, got: {msg}"
         );
     }
+
+    #[test]
+    fn parse_data_rejects_duplicate_doc_clause() {
+        let err = parse(
+            r#"
+            (module m
+              (data D
+                (doc "first")
+                (doc "second")
+                (ctor C)))
+            "#,
+        )
+        .unwrap_err();
+        let msg = format!("{err:?}");
+        assert!(
+            msg.contains("data `D` has duplicate `(doc ...)` clause"),
+            "expected duplicate-doc diagnostic, got: {msg}"
+        );
+    }
 }