There are different possible causes.
- You sent a text message that exceeded 160 characters. This is too long for a single text message and is charged as different text messages.
- You sent a text message with special characters (such as ê, ë, 'smileys’, etc.).
- Your smartphone had a technical problem and sent the same text message several times. If this is the case, have your smartphone looked at or the problem might occur again.