The error message you're receiving is correct because you are using the "invoice" variable in your donation item button code. The value of the variable is static meaning you are using the same value each time someone chooses the option.
Because the system recognizes that the invoice has already been paid, should a second customer try to purchase the same item, or in your scenario as you are using a static value, if a donor choose the same option as a previous donor, he or she will get the following message: "This invoice has already been paid. For more information, please contact the merchant".
Note:
You can turn this checking feature off in your PayPal account:
Login to your Account.
Select Profile
Under My Selling Tools
Look for: Block Payments, select Update
On the Payment Receiving Preferences page.
Look for Block accidental payments.
If you turn this off, then you can use the "invoice" variable value in your button code. Else simply remove the "invoice" variable and value line of code from your donation item button and leave the default setting to "on".
If you were using dynamic values for the "invoice" variable, then you would have a unique invoice value each time but, this would require a bit of custom scripting on your end.
