Duplicate orders only when email is turned ON (using moneyorder module)

We are having orders duplicated after they are submitted.

I think it's tied to the email being turned on (or off).

If email is turned on, orders process, get duplicated, and the confirmation email is sent, linked to the second of the two orders in the system.

If email is turned off, orders don't get duplicated, but obviously no emails are sent. This also makes our Contact Us page defunct.

Awhile back, I read somewhere that email processing is part of the checkout procedure and if the emails fail, the checkout might not complete properly. This seems to be the case.

has anyone any advice on how to troubleshoot this properly and find a solution? It just started this morning and as far as we can tell, none of our email settings - either in Zen-cart or locally on the email server - have changed.

By the way, I've seen a few threads on this subject, but but most of them have had to do with payment modules being tied into the checkout process, which we don't use. (We are a wholesale business and we process payments completely outside of zen-cart, so we just have the Check payment module modified to display "method on account" and that's the only option anyone has...)

Thanks much for any help.

By the way, email in my zencart is set to smtpauth.

Also, we tried turning email on and setting the "Send Copy of Order Confirmation Emails To" field blank, but got the same errors as above.

Using zencart 1.3.8a, php 5.2.6, PHP memory limit 32MB

Are you able to replicate the problem yourself? ie: consistently, on demand?
Or is it just something that's happening with intermittent orders?

I'm guessing that maybe your email server is very slow in responding to attempts to connect and send messages, and thus your customers are being forced to wait while that takes place, and they end up clicking the Submit button a second time, thus aborting the first order attempt partway through, and then only completing the second one, thus the emails only showing the second link, not the first.