How to prevent user to insert more then a Trading Partner Item cross reference for an Item Class

Summary:

Business would like to prevent the user to insert more then a Trading Partner Item cross reference relationship for the same Item.

The system should trigger an error or a warning message in case the user insert, for a specific Item Class, more than one Trading Partner Item Cross reference for the same item.

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Version (include the version you are using, if applicable):

26A

Code Snippet (add any code snippets that support your topic, if applicable):