Move duplicate reduction

getEthernetIfaceData callback should ideally be returning exactly the
values the system has, minus duplicates.  In every case this function is
used, we don't want duplicates, so move where we check for duplicates.

Tested: In conjunction with
https://gerrit.openbmc.org/c/openbmc/bmcweb/+/54829

 test cases pass.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ib202db99f70e1a6fdddf18969e15b2382e287848