feat: Attempt to detect already decrypted messages

lib/src/errors.dart

@@ -40,3 +40,9 @@ class InvalidKeyExchangeException implements Exception {
   final int actualPn;
   String errMsg() => 'The pn attribute of the key exchange is invalid. Expected $expectedPn, got $actualPn';
 }
+
+/// Triggered by the Session Manager when a message's sequence number is smaller than we
+/// expect it to be.
+class MessageAlreadyDecryptedException implements Exception {
+  String errMsg() => 'The message has already been decrypted';
+}

lib/src/omemo/sessionmanager.dart

@@ -401,6 +401,12 @@ class OmemoSessionManager {
     final ratchet = (await _getRatchet(ratchetKey))!;
     oldRatchet ??= ratchet.clone();
 
+    if (!rawKey.kex) {
+      if (message.n! < ratchet.nr - 1) {
+        throw MessageAlreadyDecryptedException();
+      }
+    }
+    
     try {
       if (rawKey.kex) {
         keyAndHmac = await ratchet.ratchetDecrypt(message, authMessage.writeToBuffer());

test/omemo_test.dart

@@ -730,7 +730,9 @@ void main() {
           msg.encryptedKeys,
         );
         expect(true, false);
-      } catch (_) {
+      } on MessageAlreadyDecryptedException catch (_) {
+        errorCounter++;
+      } on InvalidKeyExchangeException catch (_) {
         errorCounter++;
       }
     }