I have 2 button (Register Now) for CiviEvent
Two "Register Now" buttons is by design.

When you add more info to the event such as a formatted description, a picture,  a location map etc then the page can get quite large so the idea is that your users can easily get to a 'Register' button at the top and bottom of the page with minimal scrolling. 

If you really don't want both buttons you can customise the event display.  See various answers here on SE such as this