This is usually caused when the client clicks the pay now button more than once in the same session.
To prevent this Validate Merchant Reference needs to be activated.
This can be done in the Adumo web console by:
- Clicking on Payment Gateway
- Clicking on Settings
- Click on Gateway
- Click on Configure Settings
- Check the Validate Merchant Reference box
- Click Update.
Any duplicate merchant references generated by the merchant system will be blocked.
Please view attached GIF.