OK – I have just spent a couple of hours playing with this and have discovered that when I created a new form from scratch, it doesn’t duplicate. The only visual differences that I can see are:
1) For the confirmation message, I used a message and not a screen redirect; and
2) For many of the fields, I used the advanced field types instead of basic ones.
My programmers did set up a form that does have the redirect and that only submits once. And they also used the basic field type. But that form does some auto-populating behind the scenes so maybe in the process of that, it clears whatever the issue is.
It’s like the form goes through a double submit process when it duplicates; the first is correct but the second has the email address absent.
Anyway, my double submit / duplicate lead issue is resolved even though I don’t fully understand how.
Thanks