Create a field with unique values

May I ask the reasoning behind your requirement?

If you enter an Account by the same name Sugar already prompts you to let you know that a duplicate was found and requires you to take action to ignore that duplicate in order to continue creating it.

There are ways to customize the duplicate check so you could implement something similar for your Opportunities if desired.

https://support.sugarcrm.com/documentation/sugar_developer/sugar_developer_guide_13.0/architecture/duplicate_check/

We have auto generated Opportunity Names default to the concatenation of fields from the Opportunity itself. Maybe something similar could be an option for you too? You could make the name a calculated field which would make it read-only for the user, but in your case you would have to make sure you can combine values from the Opportunity into a unique name.

Or maybe you would be better off leaving the name to be more intuitive and informative for the user and add a unique Opportunity Number that can be referenced in conversations? 

https://support.sugarcrm.com/documentation/sugar_developer/sugar_developer_guide_13.1/cookbook/creating_an_auto-incrementing_field/

FrancescaS

Hi Jessica Linstrot ,

Our plug-in, Upsert Deduplicate, has an optional feature that prevents users from creating duplicate records within a given module. "Duplicate records" in this sense applies to any records that match an existing record based on criteria you define. 

If you would like a demo of how Upsert Deduplicate solves your need, please feel free to contact us!

Chris