Hello @abdulsalam​ ,

Please take a look at the following:

Automatic merges require no action on your part. When a person visits your website in a logged-out state, they’re given an Intercom cookie to uniquely identify them. Once they start a conversation or reply to a message, they become a lead. 

If they log in or sign up to your product while the Intercom cookie is present, their lead profile will be automatically merged with their user profile. 

More information can be found here.