Duplicate key issue with Custom Data Group

step 1:
http://192.168.2.9/drupal/civicrm/admin/custom/group?reset=1
click on the "New Custom Data Group" link

step 2:
http://192.168.2.9/drupal/civicrm/admin/custom/group?action=add&reset=1
create group as "Test Group"

step 3:
http://192.168.2.9/drupal/civicrm/admin/custom/group?reset=1
click on "Edit Group Title And Display Type"

step 4:
http://192.168.2.9/drupal/civicrm/admin/custom/group?action=update&reset=1&id=3
Edit theGroup Name as "Test Group Edit"

step 5:
Follow step 1, 2, 3 again
then http://192.168.2.9/drupal/civicrm/admin/custom/group?action=update&reset=1&id=4
Edit new Group Name as "Test Group Edit" (same as above)

now page shows following error
*****************************************************************************************************************
error

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

[code] => -5
[message] => DB Error: already exists
[mode] => 16
[debug_info] => UPDATE civicrm_custom_group SET domain_id = 1 , name = 'Test_Group_Edit' , title = 'Test Group Edit' , extends = 'Individual' , style = 'Inline' , collapse_display = 0 , help_pre = '' , weight = 4 , is_active = 1 WHERE civicrm_custom_group.id = 4 [nativecode=1062 ** Duplicate entry 'Test Group Edit-1' for key 2]
[type] => DB_Error
[user_info] => UPDATE civicrm_custom_group SET domain_id = 1 , name = 'Test_Group_Edit' , title = 'Test Group Edit' , extends = 'Individual' , style = 'Inline' , collapse_display = 0 , help_pre = '' , weight = 4 , is_active = 1 WHERE civicrm_custom_group.id = 4 [nativecode=1062 ** Duplicate entry 'Test Group Edit-1' for key 2]
[to_string] => [db_error: message="DB Error: already exists" code=-5 mode=callback callback=CRM_Core_Error::handle prefix="" info="UPDATE civicrm_custom_group SET domain_id = 1 , name = 'Test_Group_Edit' , title = 'Test Group Edit' , extends = 'Individual' , style = 'Inline' , collapse_display = 0 , help_pre = '' , weight = 4 , is_active = 1 WHERE civicrm_custom_group.id = 4 [nativecode=1062 ** Duplicate entry 'Test Group Edit-1' for key 2]"]
)
*****************************************************************************************************************

Instead of error showing on page, warning shold be shown on the form