1. Open the customer's invoices, transactions and credit history for the affected periods.
2. List the distinct provider transaction IDs, amounts and dates. Check whether WHMCS logged one transaction more than once or received two different payments.
3. Inspect the provider's active subscriptions or payment agreements for the customer and match their identifiers to the WHMCS services.
4. Check for manual payments made while an automatic subscription was still scheduled.
5. Review recent gateway migrations, service price changes and billing-cycle changes that may have left an old agreement active.

Once the intended agreement is identified, cancel only the redundant subscription through the supported provider or module process. Confirm the cancellation at the provider. Removing a Subscription ID from a WHMCS service record is not proof that the provider will stop sending money.

For legacy PayPal Basic subscriptions, the recurring amount and schedule may be fixed at the provider. A price or tax change can therefore require an updated agreement. Newer PayPal integrations use different payment models, so follow the documentation for the exact module.

Compare the client's credit balance with the excess payments. Decide whether the approved outcome is retained credit or a refund of a specific transaction. Do not remove credit and issue a refund independently without checking the combined effect on the account.

A duplicate-notification message for an already recorded transaction can be a normal protective response. It does not by itself prove a second charge occurred. Use the provider's actual transaction list to establish the financial event.

Update invoice emails so customers understand whether payment will be collected automatically. Verify the next renewal and record the subscription retained, the duplicate cancelled and the treatment of prior overpayments.

• Apply and review client credit in WHMCS
• Process and reconcile a WHMCS refund
• Change a WHMCS service due date without creating a billing gap
• Troubleshoot WHMCS payment callbacks and unpaid invoices

• WHMCS PayPal Basic module
• WHMCS credit balances
• WHMCS payment troubleshooting
• WHMCS: update tax rates