What causes duplicate orders to appear?

In the last week I have had two order duplicate.

Payment was taken once on both

The first order was duplicated once, the second order was duplicated 4 times.

Trying to understand where the duplication takes place. As there was only one payment taken wondering if its a Paypal error maybe sending multiple IPN signals, or could it be zen cart that is causing a problem.

Been in my current set up for nearly 9 months now with no problems. So this is only just starting to happen.

Using PayPal standard. Yes I know express is supposed to be better but standard has worked fine for me till now.

Any advice appreciated.

Cheers

Phil

Possibly not related, but I recently had one site do that several times as the hard disk on the server was dying.
A week later it completely died and the server had to be rebuilt.
Make sure you have current backups just in case.

philip937:

wondering if its a Paypal error maybe sending multiple IPN signals
Typically a result of PayPal sending IPNs that don't match up to already-processed orders, thus it deems it appropriate to treat it as a new-order notice.

What would be the cause of them doing that? Simply a PayPal error? My site send incorrect info or maybe somehow customer double / treble clicking perhaps?