diff --git a/crates/ailang-surface/src/parse.rs b/crates/ailang-surface/src/parse.rs
index c58a560..7a52956 100644
--- a/crates/ailang-surface/src/parse.rs
+++ b/crates/ailang-surface/src/parse.rs
@@ -484,19 +484,58 @@ impl<'a> Parser<'a> {
         let mut suppress: Vec<Suppress> = Vec::new();
         loop {
             match self.peek_head_ident() {
-                Some("doc") => doc = Some(self.parse_doc()?),
+                Some("doc") => {
+                    if doc.is_some() {
+                        let pos = self.peek().map(|t| t.span.start).unwrap_or(0);
+                        return Err(ParseError::Production {
+                            production: "fn-def",
+                            message: format!(
+                                "fn `{name}` has duplicate `(doc ...)` clause"
+                            ),
+                            pos,
+                        });
+                    }
+                    doc = Some(self.parse_doc()?);
+                }
                 Some("suppress") => suppress.push(self.parse_suppress_attr()?),
                 Some("type") => {
-                    let t = self.parse_type_attr()?;
-                    ty = Some(t);
+                    if ty.is_some() {
+                        let pos = self.peek().map(|t| t.span.start).unwrap_or(0);
+                        return Err(ParseError::Production {
+                            production: "fn-def",
+                            message: format!(
+                                "fn `{name}` has duplicate `(type ...)` clause"
+                            ),
+                            pos,
+                        });
+                    }
+                    ty = Some(self.parse_type_attr()?);
                 }
                 Some("params") => {
-                    let p = self.parse_params_attr()?;
-                    params = Some(p);
+                    if params.is_some() {
+                        let pos = self.peek().map(|t| t.span.start).unwrap_or(0);
+                        return Err(ParseError::Production {
+                            production: "fn-def",
+                            message: format!(
+                                "fn `{name}` has duplicate `(params ...)` clause"
+                            ),
+                            pos,
+                        });
+                    }
+                    params = Some(self.parse_params_attr()?);
                 }
                 Some("body") => {
-                    let b = self.parse_body_attr()?;
-                    body = Some(b);
+                    if body.is_some() {
+                        let pos = self.peek().map(|t| t.span.start).unwrap_or(0);
+                        return Err(ParseError::Production {
+                            production: "fn-def",
+                            message: format!(
+                                "fn `{name}` has duplicate `(body ...)` clause"
+                            ),
+                            pos,
+                        });
+                    }
+                    body = Some(self.parse_body_attr()?);
                 }
                 Some(other) => {
                     let pos = self.peek().map(|t| t.span.start).unwrap_or(0);
@@ -2260,4 +2299,98 @@ mod tests {
         };
         assert_eq!(i.doc.as_deref(), Some("instance for ints"));
     }
+
+    /// Iter 22-tidy.5: `parse_fn` rejects a duplicate `(doc ...)`
+    /// clause with a `fn-def` production diagnostic naming the
+    /// offending fn. Pre-22-tidy.5, the second clause silently
+    /// overwrote the first; this test pins the strict shape.
+    #[test]
+    fn parse_fn_rejects_duplicate_doc_clause() {
+        let err = parse(
+            r#"
+            (module m
+              (fn f
+                (doc "first")
+                (doc "second")
+                (type (fn-type (params) (ret (con Unit))))
+                (params)
+                (body (do io/print_str "x"))))
+            "#,
+        )
+        .unwrap_err();
+        let msg = format!("{err:?}");
+        assert!(
+            msg.contains("fn `f` has duplicate `(doc ...)` clause"),
+            "expected duplicate-doc diagnostic, got: {msg}"
+        );
+    }
+
+    /// Iter 22-tidy.5: `parse_fn` rejects a duplicate `(type ...)`
+    /// clause with a `fn-def` production diagnostic naming the
+    /// offending fn.
+    #[test]
+    fn parse_fn_rejects_duplicate_type_clause() {
+        let err = parse(
+            r#"
+            (module m
+              (fn f
+                (type (fn-type (params) (ret (con Unit))))
+                (type (fn-type (params) (ret (con Unit))))
+                (params)
+                (body (do io/print_str "x"))))
+            "#,
+        )
+        .unwrap_err();
+        let msg = format!("{err:?}");
+        assert!(
+            msg.contains("fn `f` has duplicate `(type ...)` clause"),
+            "expected duplicate-type diagnostic, got: {msg}"
+        );
+    }
+
+    /// Iter 22-tidy.5: `parse_fn` rejects a duplicate `(params ...)`
+    /// clause with a `fn-def` production diagnostic naming the
+    /// offending fn.
+    #[test]
+    fn parse_fn_rejects_duplicate_params_clause() {
+        let err = parse(
+            r#"
+            (module m
+              (fn f
+                (type (fn-type (params) (ret (con Unit))))
+                (params)
+                (params)
+                (body (do io/print_str "x"))))
+            "#,
+        )
+        .unwrap_err();
+        let msg = format!("{err:?}");
+        assert!(
+            msg.contains("fn `f` has duplicate `(params ...)` clause"),
+            "expected duplicate-params diagnostic, got: {msg}"
+        );
+    }
+
+    /// Iter 22-tidy.5: `parse_fn` rejects a duplicate `(body ...)`
+    /// clause with a `fn-def` production diagnostic naming the
+    /// offending fn.
+    #[test]
+    fn parse_fn_rejects_duplicate_body_clause() {
+        let err = parse(
+            r#"
+            (module m
+              (fn f
+                (type (fn-type (params) (ret (con Unit))))
+                (params)
+                (body (do io/print_str "x"))
+                (body (do io/print_str "y"))))
+            "#,
+        )
+        .unwrap_err();
+        let msg = format!("{err:?}");
+        assert!(
+            msg.contains("fn `f` has duplicate `(body ...)` clause"),
+            "expected duplicate-body diagnostic, got: {msg}"
+        );
+    }
 }