MGMT Interface on "Internal"

Stupid question that I've been beating my head against.  My new FS (1048E running v6.4.2) has a dedicated mgmt interface but I don't want to use it and would rather have mgmt allowed on any interface that is up/connected to the network and essentially has a management IP address assigned globally.  On my old switches I'd assign an IP address to a VLAN and allow mgmt protocols.  Pretty simple.  I've followed the instructions in the admin guide for both "models with dedicated" and models without but am stumbling at the part where after configuring the "internal" interface (ip address, allowed access, etc) it wants me to "create a new interface to be used for management" and assign an address to it...which it won't allow because the mgmt address is in use by the "internal" interface.

config system interface      edit internal          set ip 172.16.1.50/24          set allowaccess ping https ssh          set type physical          set secondary-IP enable               config secondaryip                   edit <id>                       set ip <IP_address_and_netmask>                       set allowaccess <access_types>                  next             end

        next edit MGMT1          set ip 172.16.1.50/24          set allowaccess ping https ssh          set interface internal          set vlanid 1          set secondary-IP enable                config secondaryip                edit <id>                set ip <IP_address_and_netmask>               set allowaccess <access_types>        end  end

that results in the obvious error of a duplicate IP.  I can't seem to turn it up unless it's on the dedicated mgmt port which I don't want. Any suggestions would be appreciated.