Consider a scenario where the user profiles are imported from Active directory in a SharePoint 2010 Environment.
CONTOSO\User1 browses to his "My Profiles" page and goes to the Colleagues tab --> Clicks on "Add Colleagues" and adds CONTOSO\User2 as his colleague. CONTOSO\User2 receives an email saying that CONTOSO\User1 added him as his colleague.
Now CONTOSO\User2 browses to his "My Profiles" page and goes to Colleagues tab --> Clicks on "Add Colleagues" and adds CONTOSO\User1 as his colleague.
However the CONTOSO\User1 never receives the notification email. The notification email is not sent when the other user is already added as a colleague.
The first time CONTOSO\user1 adds CONTOSO\user2 as a colleague we send a notification email to CONTOSO\User2, however the product is designed such that we don’t send the notification email when CONTOSO\User2 adds/confirms the colleague.