--- core/src/main/java/quickfix/Session.java (original)
+++ core/src/main/java/quickfix/Session.java
@@ -1112,8 +1112,10 @@
         }
         reject.setString(RefSeqNum.FIELD, msgSeqNum);
 
+// ry 2010-09-15: Only advance the sequence number if we are at the expected number.
+//                What happens if msgSeqNum is invalid?
         if (!msgType.equals(MsgType.LOGON) && !msgType.equals(MsgType.SEQUENCE_RESET)
-                && !isPossibleDuplicate(message)) {
+                && Integer.valueOf(msgSeqNum) == getExpectedTargetNum()) {
             state.incrNextTargetMsgSeqNum();
         }
 
@@ -1172,8 +1174,9 @@
 
         state.lockTargetMsgSeqNum();
         try {
+// ry 2010-09-15: Only advance the sequence number if we are at the expected number.
             if (!msgType.equals(MsgType.LOGON) && !msgType.equals(MsgType.SEQUENCE_RESET)
-                    && (msgSeqNum == getExpectedTargetNum() || !isPossibleDuplicate(message))) {
+                    && msgSeqNum == getExpectedTargetNum()) {
                 state.incrNextTargetMsgSeqNum();
             }
         } finally {