It's normal to say 'Nice to meet you' you for the first time, and 'Nice to see you' for subsequent meetings. 'Meet' in this sense means 'make your acquaintance' [fare la conoscenza] which - logically speaking - you can only do once.
However, if you were to say 'Nice to meet you again' on the occasion of a second meeting, this wouldn't be seen as a mistake. You could interpret this as the other meaning of meet [ incontrarsi], which would be fine.