I've attached a screen grab of the Server Behaviors panel when I open your page in Dreamweaver.
You can see that the first and fourth item listed are both "WA Server Validations"
That means you added it twice. You should have edited the existing one to add another validated field (CAPTCHA) where the other three validations are instead of adding a new Validation server behavior.
Now you can delete the first one and edit the second one listed to add the captcha there instead.