Earlier to “Alternate Keys”, for Duplicate Record detections developer either rely on Duplicate Detection rules or write Custom Plugin. Each of it has its own advantages and disadvantages.
Duplicate Detection rules are codeless and quick to set up however they do not block users by creating Duplicate records, users can accept the warning and proceed to create duplicate records.
Custom Plugins, need custom coding and and it will completely block users from creating duplicate records, however its not flexible as Duplicate Detection rules.
With invent of “Alternate Keys”, we can use it in place of Custom Plugins and we still do not have to create custom plugins. Once Alternate Keys are define CRM platform restrict users from creating duplicate records
Only disadvantage of it is you can have Custom Error Messages which will describe the error in User friendly way
Following is the message that is thrown by CRM