Reject duplicate Kiwi field IDs

Found while testing crates/format/src/fig/kiwi.rs:615

Co-Authored-By: GPT-5.6-Luna <noreply@openai.com>
Co-Authored-By: GPT-6-Luna <noreply@openai.com>

crates/format/src/fig/kiwi.rs

@@ -270,6 +270,9 @@ impl Schema {
                 let ty = r.var_int()?;
                 let array = r.byte()? & 1 != 0;
                 let value = r.var_uint()?;
+                if kind == Kind::Message && fields.iter().any(|f: &Field| f.value == value) {
+                    return err(format!("schema: duplicate field id {value} in {name}"));
+                }
                 raw_types.push(ty);
                 fields.push(Field { name: fname, ty: None, array, value });
             }
@@ -621,6 +624,9 @@ impl Schema {
                         }
                         .ok_or_else(|| KiwiError(format!("schema text: unknown type {base}")))?;
                         let (name, value) = numbered(n.trim(), id)?;
+                        if *kind == "message" && fields.iter().any(|f: &Field| f.value == value) {
+                            return err(format!("schema text: duplicate field id {value} in {name}"));
+                        }
                         fields.push(Field { name, ty: Some(ty), array: t.ends_with("[]"), value });
                     }
                     (if *kind == "struct" { Kind::Struct } else { Kind::Message }, fields)

crates/format/src/fig/tests.rs

@@ -562,6 +562,11 @@ fn oversized_grid_track_lists_warn_and_use_default_tracks() {
     assert!(warnings.iter().any(|w| w.contains("grid track list exceeds 1000 entries")), "{warnings:?}");
 }
 
+#[test]
+fn duplicate_explicit_field_ids_are_ambiguous() {
+    assert!(Schema::parse("message M uint a=1, uint b=1").is_err());
+}
+
 #[test]
 fn kiwi_round_trip() {
     let sc = schema();