With what you have currently provided, there is not enough information to indicate what's causing the problem.
Is email.php being called twice? You can check the web browser network diagnostics to determine that.
If it's not being called twice, then the double email is occuring from somewhere on the server side.
If it is being called twice, then it's possible that the script issuing an ajax request, and the default behaviour of the form is occurring too.
In order to work out where you need to investigate, we need more information about what is causing the problem.
So either you can provide that information to us, or you can provide a test page that demonstrates the problem, so that we can investigate things further.