Index: core/src/main/java/quickfix/DataDictionary.java
===================================================================
--- core/src/main/java/quickfix/DataDictionary.java (rev1)
+++ core/src/main/java/quickfix/DataDictionary.java
@@ -71,6 +71,7 @@
     private boolean checkFieldsOutOfOrder = true;
     private boolean checkFieldsHaveValues = true;
     private boolean checkUserDefinedFields = true;
+    private boolean checkDuplicateFields = true;
     private boolean allowUnknownMessageFields = false;
     private String beginString;
     private final Map<String, Set<Integer>> messageFields = new HashMap<String, Set<Integer>>();
@@ -486,12 +487,22 @@
         checkUserDefinedFields = flag;
     }
 
+    public boolean isCheckDuplicateFields() {
+        return checkDuplicateFields;
+    }
+
+    public void setCheckDuplicateFields(boolean checkDuplicateFields) {
+        this.checkDuplicateFields = checkDuplicateFields;
+    }
+
     private void copyFrom(DataDictionary rhs) {
         hasVersion = rhs.hasVersion;
         beginString = rhs.beginString;
         checkFieldsOutOfOrder = rhs.checkFieldsOutOfOrder;
         checkFieldsHaveValues = rhs.checkFieldsHaveValues;
         checkUserDefinedFields = rhs.checkUserDefinedFields;
+        checkDuplicateFields = rhs.checkDuplicateFields;
+        allowUnknownMessageFields = rhs.allowUnknownMessageFields;
 
         copyMap(messageFields, rhs.messageFields);
         copyMap(requiredFields, rhs.requiredFields);
Index: core/src/main/java/quickfix/Message.java
===================================================================
--- core/src/main/java/quickfix/Message.java (original)
+++ core/src/main/java/quickfix/Message.java
@@ -541,24 +541,25 @@
             if (isHeaderField(field.getField())) {
                 // An acceptance test requires the sequence number to
                 // be available even if the related field is out of order
-                setField(header, field);
-                throw new FieldException(SessionRejectReason.TAG_SPECIFIED_OUT_OF_REQUIRED_ORDER,
-                        field.getTag());
-            }
-
-            setField(this, field);
-
-            // Group case
-            if (dd != null && dd.isGroup(getMsgType(), field.getField())) {
-                parseGroup(getMsgType(), field, dd, this);
+                setField(header, field, dd);
+                if(dd.isCheckFieldsOutOfOrder())
+                    throw new FieldException(SessionRejectReason.TAG_SPECIFIED_OUT_OF_REQUIRED_ORDER,
+                            field.getTag());
+            } else {
+                setField(this, field, dd);
+    
+                // Group case
+                if (dd != null && dd.isGroup(getMsgType(), field.getField())) {
+                    parseGroup(getMsgType(), field, dd, this);
+                }
             }
 
             field = extractField(dd, this);
         }
     }
 
-    private void setField(FieldMap fields, StringField field) {
-        if (fields.isSetField(field)) {
+    private void setField(FieldMap fields, StringField field, DataDictionary dd) {
+        if (dd.isCheckDuplicateFields() &&  fields.isSetField(field)) {
             throw new FieldException(SessionRejectReason.TAG_APPEARS_MORE_THAN_ONCE, field.getTag());
         }
         fields.setField(field);
Index: core/src/main/java/quickfix/Session.java
===================================================================
--- core/src/main/java/quickfix/Session.java (rev1)
+++ core/src/main/java/quickfix/Session.java
@@ -206,6 +206,11 @@
     public static final String SETTING_VALIDATE_USER_DEFINED_FIELDS = "ValidateUserDefinedFields";
 
     /**
+     * Controls validation of duplicate fields.
+     */
+    public static final String SETTING_VALIDATE_DUPLICATE_FIELDS = "ValidateDuplicateFields";
+
+    /**
      * Session setting that causes the session to reset sequence numbers when initiating
      * a logon (>= FIX 4.2).
      */
@@ -2063,4 +2068,4 @@
     public ApplVerID getTargetDefaultApplicationVersionID() {
         return targetDefaultApplVerID.get();
     }
-}
+}
\ No newline at end of file