diff --git a/parser/grammar.peg b/parser/grammar.peg
index 53a9947..1bed281 100644
--- a/parser/grammar.peg
+++ b/parser/grammar.peg
@@ -177,15 +177,31 @@ StructLike ← name:Identifier __ '{' __ fields:FieldList '}' EOS {
 FieldList ← fields:(Field __)* {
 	fs := fields.([]interface{})
 	flds := make([]*Field, len(fs))
+	ids := make(map[int]bool, len(fs))
 	for i, f := range fs {
 		flds[i] = f.([]interface{})[0].(*Field)
+
+		if ids[flds[i].ID] {
+			return nil, fmt.Errorf("multiple fields with the same identifier: %d", flds[i].ID)
+		}
+
+		ids[flds[i].ID] = true
 	}
 	return flds, nil
 }
 
-Field ← id:IntConstant _ ':' _ req:FieldReq? _ typ:FieldType _ name:Identifier __ def:('=' _ ConstValue)? ListSeparator? {
+FieldID ← id:IntConstant _ ':' {
+	return int(id.(int64)), nil
+}
+
+Field ← id:FieldID? _ req:FieldReq? _ typ:FieldType _ name:Identifier __ def:('=' _ ConstValue)? ListSeparator? {
+	parsedID := -1
+	if id != nil {
+		parsedID = id.(int)
+	}
+
 	f := &Field{
-		ID       : int(id.(int64)),
+		ID       : parsedID,
 		Name     : string(name.(Identifier)),
 		Type     : typ.(*Type),
 	}