Check for duplicate adds

app/src/main/java/eu/faircode/email/Core.java

@@ -1352,6 +1352,13 @@ class Core {
                 try {
                     db.beginTransaction();
 
+                    // Check if message was added in the meantime
+                    EntityMessage existing = db.message().getMessageByUid(message.folder, message.uid);
+                    if (existing != null) {
+                        Log.i("Message was already added");
+                        return existing;
+                    }
+
                     message.id = db.message().insertMessage(message);
                     Log.i(folder.name + " added id=" + message.id + " uid=" + message.uid);