Hello! Newbie, so if this is a well-known issue, I apologize. I searched the archives and found a lot of irrelevant hits; nothing that matched.

While testing my system, I created a user account, and placed an order. Yea! So far, so good.

Place another one, and always get an error message:

This appears to be a duplicate transaction as it is an order for the same order amount in a short period of time. Your credit card has not been charged. Please check the order history, and resubmit in 5 minutes if this is a valid order.
Here are the problems:

This appears to be a duplicate transaction as it is an order for the same order amount in a short period of time.
Not a duplicate. Different items ordered. Also, "short period of time" = 24 hours or so.

resubmit in 5 minutes if this is a valid order.
OK, but that doesn't do any good. Keep hitting the same error over and over again, until it times out and bumps me out.

Then it tells me to log in again, and my order will be there. But it's not. Start all over--and get the same error message again! Even ordering different item in different quantities.

Much obliged for insights. I have looked for any settings that I might be able to tweak, but I have not found any relevant ones yet.

Unless the orders are within seconds, I would not expect the system would call them duplicates. And to the extent it does, I as the "customer" should be able to over-ride and order it anyway. There does not seem to be an obvious way of doing that.

My customers do place new orders, often fairly quickly after placing the last one. My inventory is rare merchandise, and they are collectors. Often notice something they want after placing an order, or they decide they want more of something, or I add more inventory to what we have, and the order more. This will be frustrating for them and I will lose orders. Maybe other people don't get as much repeat business so they don't notice, or maybe I am just unlucky to have stumbled across an obscure bug.

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What version of Zen Cart are you using?

1.5.0

Has your site been upgraded? When? From what version(s)?

No.

What plugins/addons have you installed? When?

Intuit Merchant Services addon. Unlikely to be relevant.

HOW did you install Zen Cart? (upload via FTP and run zc_install, or a one-click install from your hosting company? (one-click installs might mean you don't have enough information about your site/server to make customizations easily))

One-click from hosting company.

What version of PHP and MySQL is your server using? (See Admin->Tools->Server Info)
Server OS: Linux 3.2.31 Database: MySQL 5.5.23-55
PHP Version: 5.2.17 (Zend: 2.2.0)

WHEN did the problem "start"?

As soon as I started testing newly installed system. Immediately.

How does that compare with other events of your hosting company, changes you've made to your site files/addons, or your admin settings, etc?

If this is your first store, have you tested all aspects of transactions before going live?

Yes; that's how I discovered the problem.

In what ways is your site customized or different from a brand new uncustomized install?

Have Intuit Merchant services addon installed. But it is unlikely to impact duplicate order detection.

HAVE YOU LOOKED IN THE FAQ AREA for answers to your question? (ie: a search for your error message or what you want to edit/change, etc)

Yes.

Have you searched the FORUM for your error message or for answers to the question you're asking?

Yes. All hits appear to be irrelevant.