in WHMCS v6.x was added transaction validation in admin area -> Invoice -> Add Payment
and some time we have same transaction ID from bank transfer or Credit Card gateway manual transactions.
this make the work in billing to less efficient and more cumbersome.
Please add some option in Setup -> General Settings -> Invoices that say "Disable Transaction ID validation"
selection of this option will disable this Transaction ID validation and allow to use it as it's was before this new validation.
Unique Invoice Id change for Payment gateway attempt
Dear Team,Most Payment gateways are not allowed Duplicate Order Id Process. Right Now, Once getting failed transaction response from payment gateway after unable to process next attempt. it show error message duplicate order id error. So Please consider upcoming version to add this feature.Thanks,M.Senthil Kumar
As noted above, this has been implemented in WHMCS 7.2.0.
Have a great day,