importing leads in overide mode

importing leads in overide mode

I should be able to select what field i want to check against when importing leads in overide mode.  I don't always have an e-mail for leads being created and so i have no way to import info for these leads without duplicating all of them.  I don't know how hard it would be to allow me to select any field in the crm to check against but this is something i would like to see.