# UNDF: UNDF-2026-000000301
--- a/storm-client/src/jvm/org/apache/storm/tuple/Fields.java
+++ b/storm-client/src/jvm/org/apache/storm/tuple/Fields.java
@@ -35,12 +35,14 @@
     public Fields(List<String> fields) {
         this.fields = new ArrayList<>(fields.size());
         for (String field : fields) {
-            if (this.fields.contains(field)) {
+            if (index.containsKey(field)) {
                 throw new IllegalArgumentException(
                     String.format("duplicate field '%s'", field)
                 );
             }
             this.fields.add(field);
+            index.put(field, this.fields.size() - 1);
         }
-        index();
+        // index already fully populated above; no-op call removed
     }