Re-add check for duplicate instance IP

This was originally implemented in 0ce8f948 and partially
rolled back in 9b65e0d4. Apart from re-adding the check,
this patch does some housekeeping by renaming the “_helper”
function to “_AddIpAddress”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>