I've seen this issue posted in a couple other places and was unable to
find the answer myself. It's an issue that has been driving me nuts for
years and I just solved it. I could never figure out why some of my
forms would open the confirmation page in a separate window while
others opened it in the same window.
Here's the simple scoop: if your form is an include on another page,
the confirmation will open in a new window. If your form is not an
include, it will open the confirmation on the same page.
I hope this thread shows up in a Google search for someone in the
future. I could have used it myself a few times.