diff --git a/crates/format/src/fig/interop_tests.rs b/crates/format/src/fig/interop_tests.rs
index fa95d792b..4ad81cebc 100644
--- a/crates/format/src/fig/interop_tests.rs
+++ b/crates/format/src/fig/interop_tests.rs
@@ -403,10 +403,10 @@ fn the_schema_numbers_what_the_format_numbers() {
 
 #[test]
 fn schema_text_takes_numbers_and_refuses_bad_ones() {
-    let s = Schema::parse("enum E A=3 B C=9\nmessage M uint a=7, E b, string c=2").unwrap();
+    let s = Schema::parse("enum E A=3 B C=9\nmessage M uint a=7, E b, string c=5").unwrap();
     let values = |i: usize| s.defs[i].fields.iter().map(|f| (f.name.as_str(), f.value)).collect::<Vec<_>>();
     assert_eq!(values(0), [("A", 3), ("B", 1), ("C", 9)]);
-    assert_eq!(values(1), [("a", 7), ("b", 2), ("c", 2)]);
+    assert_eq!(values(1), [("a", 7), ("b", 2), ("c", 5)]);
     for bad in ["enum E A=", "enum E A=x", "enum E A=-1", "message M uint a=1.5", "message M uint a=99999999999", "enum E =", "message M uint ="] {
         assert!(Schema::parse(bad).is_err() || bad.ends_with('='), "{bad}");
     }
diff --git a/crates/format/src/fig/kiwi.rs b/crates/format/src/fig/kiwi.rs
index 909dad87c..046fcaa84 100644
--- a/crates/format/src/fig/kiwi.rs
+++ b/crates/format/src/fig/kiwi.rs
@@ -276,6 +276,9 @@ 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 });
             }
@@ -645,6 +648,9 @@ 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)
diff --git a/crates/format/src/fig/tests.rs b/crates/format/src/fig/tests.rs
index 802a8c113..883d5385e 100644
--- a/crates/format/src/fig/tests.rs
+++ b/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();