How to populate warning message through sandbox if multiple project code entered in requisition page

Summary:

We have a requirement not to allow requisition lines with multiple project, one requisition one project code is the objective, however, requisition page allows to select multiple projects in multiple lines, is there a way to control entering requisition lines with different project number and is there a way to populate warning message if multiple project entered in a requisition.

Content (please ensure you mask any confidential information):

We have a requirement not to allow requisition lines with multiple project, one requisition one project code is the objective, however, requisition page allows to select multiple projects in multiple lines, is there a way to control entering requisition lines with different project number and is there a way to populate warning message if multiple project entered in a requisition.