Why have I been charged twice for sending one message?
In most cases, this will be caused by your content exceeding 160 characters, the character limit for one SMS. It is possible to send multiple SMS together, formed as a single, longer SMS; this is called a Concatenated SMS. A one-part message will allow you to send up to 160 characters, a two-part messages will allow you to send up to 206 characters and a three-part message will allow you to send up to 459 characters.
To join messages together to ensure they deliver as one, a 7 byte concatenation header is used at the beginning of each SMS. Since this header is within the message, it reduces the total size of each SMS to 153 characters each.
Please also be aware that if you are sending in Unicode, your character count per SMS lowers to 70 characters for a one-part message due to the fact that each UCS2 character requires 2 bytes. The 7 byte header for concatenated messages will also apply sending in excess of one message. As a result, a two-part Unicode message has a limit of 134 characters and a three-part Unicode message has a limit of 201 characters.