Plugin duplicating transactions.

[imagina]

It's not checking for transaction already in the system.

You should add :

checkCbTransID($validationData['x_ref_payco']); # Checks transaction number isn't already in the database and ends processing if it does

Docs: https://developers.whmcs.com/payment-gateways/callbacks/

[fabolivar]

Same problem, any update?

[RonaldRengifoB]

This issue is still replicating up to the latest version (Dec 2025), I talked to WHMCS support and they replied:
Looking at the Gateway Logs in your WHMCS installation (Billing > Gateway Logs) it shows that the payment gateway sent two callbacks following payment, where we would normally only expect one. Therefore WHMCS added two transactions. You may wish to contact the payment provider to ascertain why the callback was sent twice and perhaps contact the payment module provider.
I've sent an email to desarrollo@epayco.com.