When you try to save a relationship whose customer opportunity role duplicates an existing relationship in Microsoft Dynamics CRM 4.0, you receive the following error message as expected:
The relationship already exists. To create a new relationship, select a different opportunity or customer, or change the customer's role.
However, after you change the customer opportunity role and try to save the relationship, you still receive the error message.
This problem is fixed in the latest cumulative update rollup for Microsoft Dynamics CRM 4.0. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
Microsoft Dynamics CRM 4.0 updates and hotfixes