diff --git a/node/src/main/kotlin/net/corda/node/services/messaging/P2PMessageDeduplicator.kt b/node/src/main/kotlin/net/corda/node/services/messaging/P2PMessageDeduplicator.kt
index becc12e116..e96b832933 100644
--- a/node/src/main/kotlin/net/corda/node/services/messaging/P2PMessageDeduplicator.kt
+++ b/node/src/main/kotlin/net/corda/node/services/messaging/P2PMessageDeduplicator.kt
@@ -62,8 +62,7 @@ class P2PMessageDeduplicator(cacheFactory: NamedCacheFactory, private val databa
 
     private fun isDuplicateInDatabase(msg: ReceivedMessage): Boolean = database.transaction { msg.uniqueMessageId.sessionIdentifier in sessionData }
 
-    // We need to incorporate the sending party, and the sessionInit flag as per the in-memory cache.
-    private fun senderHash(senderKey: SenderKey) = SecureHash.sha256(senderKey.peer.toString() + senderKey.isSessionInit.toString() + senderKey.senderUUID).toString()
+    private fun senderHash(senderKey: SenderKey) = SecureHash.sha256(senderKey.peer.toString() + senderKey.senderUUID).toString()
 
     /**
      * Determines whether a session-init message is a duplicate.
@@ -85,10 +84,12 @@ class P2PMessageDeduplicator(cacheFactory: NamedCacheFactory, private val databa
      * Called the first time we encounter [deduplicationId].
      */
     fun signalMessageProcessStart(msg: ReceivedMessage) {
+        require(msg.uniqueMessageId.messageType == MessageType.SESSION_INIT) { "Message ${msg.uniqueMessageId} was not a session-init message." }
+
         val receivedSenderUUID = msg.senderUUID
         val receivedSenderSeqNo = msg.senderSeqNo
         // We don't want a mix of nulls and values so we ensure that here.
-        val senderHash: String? = if (receivedSenderUUID != null && receivedSenderSeqNo != null) senderHash(SenderKey(receivedSenderUUID, msg.peer, msg.isSessionInit)) else null
+        val senderHash: String? = if (receivedSenderUUID != null && receivedSenderSeqNo != null) senderHash(SenderKey(receivedSenderUUID, msg.peer)) else null
         val firstSenderSeqNo: Long? = if (senderHash != null) msg.senderSeqNo else null
         beingProcessedMessages[msg.uniqueMessageId] = MessageMeta(msg.uniqueMessageId.timestamp, senderHash, firstSenderSeqNo, null)
     }
@@ -97,6 +98,8 @@ class P2PMessageDeduplicator(cacheFactory: NamedCacheFactory, private val databa
      * Called inside a DB transaction to persist [deduplicationId].
      */
     fun persistDeduplicationId(deduplicationId: MessageIdentifier) {
+        require(deduplicationId.messageType == MessageType.SESSION_INIT) { "Message $deduplicationId was not a session-init message." }
+
         sessionData[deduplicationId.sessionIdentifier] = beingProcessedMessages[deduplicationId]!!
     }
 
@@ -105,6 +108,8 @@ class P2PMessageDeduplicator(cacheFactory: NamedCacheFactory, private val databa
      * Any subsequent redelivery will be deduplicated using the DB.
      */
     fun signalMessageProcessFinish(deduplicationId: MessageIdentifier) {
+        require(deduplicationId.messageType == MessageType.SESSION_INIT) { "Message $deduplicationId was not a session-init message." }
+
         beingProcessedMessages.remove(deduplicationId)
     }
 
@@ -173,5 +178,5 @@ class P2PMessageDeduplicator(cacheFactory: NamedCacheFactory, private val databa
 
     private data class MessageMeta(val generationTime: Instant, val senderHash: String?, val firstSenderSeqNo: SenderSequenceNumber?, val lastSenderSeqNo: SenderSequenceNumber?)
 
-    private data class SenderKey(val senderUUID: String, val peer: CordaX500Name, val isSessionInit: Boolean)
+    private data class SenderKey(val senderUUID: String, val peer: CordaX500Name)
 }