Who is your host?
Are they using an Exchange email server?
open the server behaviors panel, and double click the Universal Email server behavior. to set a thank you redirect, you can set the Go To page.
click on the To button to go to the Advanced to settings.
try increasing the time between emails. if the host is using an exchange server, it may send duplicate emails if it tries to send multiple copies to different addresses, increasing the time between emails can correct that.