Why is the printer printing double?

If the printer is printing the same document multiple times, it's because there are multiple instances of the Mews Connector application running simultaneously.

When a command is issued to the printer, every instance of the Mews Connector application receives the command and process it, which then results in two or more copies of the same document. 

If you are experiencing an issue with multiple instances of Mews Connector, check this article: Duplicate Mews Connector