diff --git a/crypto/sql_store.go b/crypto/sql_store.go
index 13940d79..ca75b3f6 100644
--- a/crypto/sql_store.go
+++ b/crypto/sql_store.go
@@ -664,6 +664,20 @@ func (store *SQLCryptoStore) IsOutboundGroupSessionShared(ctx context.Context, u
 // ValidateMessageIndex returns whether the given event information match the ones stored in the database
 // for the given sender key, session ID and index. If the index hasn't been stored, this will store it.
 func (store *SQLCryptoStore) ValidateMessageIndex(ctx context.Context, senderKey id.SenderKey, sessionID id.SessionID, eventID id.EventID, index uint, timestamp int64) (bool, error) {
+	if eventID == "" && timestamp == 0 {
+		var notOK bool
+		const validateEmptyQuery = `
+		SELECT EXISTS(SELECT 1 FROM crypto_message_index WHERE sender_key=$1 AND session_id=$2 AND "index"=$3)
+		`
+		err := store.DB.QueryRow(ctx, validateEmptyQuery, senderKey, sessionID, index).Scan(&notOK)
+		if notOK {
+			zerolog.Ctx(ctx).Debug().
+				Uint("message_index", index).
+				Msg("Rejecting event without event ID and timestamp due to already knowing them")
+		}
+		return !notOK, err
+	}
+
 	const validateQuery = `
 	INSERT INTO crypto_message_index (sender_key, session_id, "index", event_id, timestamp)
 	VALUES ($1, $2, $3, $4, $5)
diff --git a/crypto/store.go b/crypto/store.go
index 8b7c0a96..7620cf35 100644
--- a/crypto/store.go
+++ b/crypto/store.go
@@ -525,6 +525,9 @@ func (gs *MemoryStore) ValidateMessageIndex(_ context.Context, senderKey id.Send
 	}
 	val, ok := gs.MessageIndices[key]
 	if !ok {
+		if eventID == "" && timestamp == 0 {
+			return true, nil
+		}
 		gs.MessageIndices[key] = messageIndexValue{
 			EventID:   eventID,
 			Timestamp: timestamp,
diff --git a/crypto/store_test.go b/crypto/store_test.go
index 8aeae7af..7a47243e 100644
--- a/crypto/store_test.go
+++ b/crypto/store_test.go
@@ -75,8 +75,13 @@ func TestValidateMessageIndex(t *testing.T) {
 		t.Run(storeName, func(t *testing.T) {
 			acc := NewOlmAccount()
 
+			// Validating without event ID and timestamp before we have them should work
+			ok, err := store.ValidateMessageIndex(context.TODO(), acc.IdentityKey(), "sess1", "", 0, 0)
+			require.NoError(t, err, "Error validating message index")
+			assert.True(t, ok, "First message validation should be valid")
+
 			// First message should validate successfully
-			ok, err := store.ValidateMessageIndex(context.TODO(), acc.IdentityKey(), "sess1", "event1", 0, 1000)
+			ok, err = store.ValidateMessageIndex(context.TODO(), acc.IdentityKey(), "sess1", "event1", 0, 1000)
 			require.NoError(t, err, "Error validating message index")
 			assert.True(t, ok, "First message validation should be valid")
 
@@ -94,6 +99,11 @@ func TestValidateMessageIndex(t *testing.T) {
 			ok, err = store.ValidateMessageIndex(context.TODO(), acc.IdentityKey(), "sess1", "event1", 0, 1000)
 			require.NoError(t, err, "Error validating message index")
 			assert.True(t, ok, "First message validation should be valid")
+
+			// Validating without event ID and timestamp must fail if we already know them
+			ok, err = store.ValidateMessageIndex(context.TODO(), acc.IdentityKey(), "sess1", "", 0, 0)
+			require.NoError(t, err, "Error validating message index")
+			assert.False(t, ok, "First message validation should be invalid")
 		})
 	}
 }