Validation functionality in service entry sheet

Hi,

We have made provision to use field - External service no- to manually input some number for reference in the line item tab in ML81N. This number is used to track LR no for internal usage. There will be s separate number in each line item. Since this is a text field, system does not check for duplicate entries.

We want a functionality to validate already entered number. If the number is repeated, system should not allow it.

Please suggest.

Regards,

Rutwik