Putting the same friend twice on the tell a friend form causes a fatal error

Here are the details of the error message:

Database Error Code: Duplicate entry '104-549' for key 'UI_activity_target_contact_id', 1062
Additional Details:

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] => -5
[message] => DB Error: already exists
[mode] => 16
[debug_info] => INSERT INTO civicrm_activity_target (activity_id , target_contact_id ) VALUES ( 549 , 104 ) [nativecode=1062 ** Duplicate entry '104-549' for key 'UI_activity_target_contact_id']
[type] => DB_Error
[user_info] => INSERT INTO civicrm_activity_target (activity_id , target_contact_id ) VALUES ( 549 , 104 ) [nativecode=1062 ** Duplicate entry '104-549' for key 'UI_activity_target_contact_id']
[to_string] => [db_error: message="DB Error: already exists" code=-5 mode=callback callback=CRM_Core_Error::handle prefix="" info="INSERT INTO civicrm_activity_target (activity_id , target_contact_id ) VALUES ( 549 , 104 ) [nativecode=1062 ** Duplicate entry '104-549' for key 'UI_activity_target_contact_id']"]
)

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