I've had numerous customer's incur the following when using PayPal:

After clicking the Confirm Order button, the page refreshes and nothing happens (they are still on step 3 of 3). So they click confirm order again, and same thing occurs.

Therefore, two things can happen:
Sometimes they think their order went thru (when in fact it did not)
OR
They realize it did not go thru and get frustrated. (because they do not see any error messages).

Then they are concerned of "Did my order go thru a bunch of times and was I charged multiple times?"

From what I can tell this usually happens when there was an error w/ attempting to process the payment on the PayPal end. Do the error messages from PayPal (eg. AVS mismatch, etc) not get relayed back to the customer?