As I understand from your post, this is a web application that you are using, and someone else have developed it. It's really little you can do but contacting the company that sold the solution to you. This appears to be a bug in the application (either client-side or in the database).
This error indicates a flaw in the business logic of the application. The database is set up to allow only a single instance of a particular value in the specified table, and the interface is attempting to insert a second instance of that value.
Its kind of like if the government tried to assing your SSN to another person as well as you. It would cause all sorts of problems. Same in the application, so the database does not allow it.
If it's not practically useful, then it's practically useless.