Can anyone describe how to set up a layout for center fed #10 envelopes?😠 FM help says that after creating the layout you must verify margins and other settings before printing. How do you do that? I've tried everything I can think of, but the script  print step prints 2 or 3 envelopes when the print dialog is set to 1 copy. I've tried adjusting the header and body dimensions (by guessing),  I've tried deleting the header and keeping the body, I've even tried just keeping the header. Everything looks normal in Preview.

Can you show a screenshot of your layout, in layout mode, showing margins/pagebreaks?

Usually the problem is if a part that prints is outside the margin or page break, it will print another (page).

For checking, without wasting paper/envelopes, isolate the record (found count of 1), then select preview mode.  See how many pages it says there are for that preview (in the toolbar or CTRL + Down Arrow).  It's correct when there is only one.

Make sure there are no hidden objects or incorrect sliding objects on (or even off) of the layout.

Here's a pic of mine.  

Attached is a picture of the layout. Print margins are fixed: 12 pt (tob, btm; left and right). Print preview shows 2 pages. There are no other objects on the layout. Print Setup is set to landscape.

I would just make the layout smaller.  Your margins may be bigger than the default or set margins from the print driver