Mirek means Automatic static NAT, which does not make much sense as it will create 4 entries in the NAT rules, 2 inbound and 2 outbound, only the first of the 2 inbound will be allowed. So if it does no longer give a warning, when trying to assign a duplicate automatic static NAT, that is a shortcoming.
When I need to use a outbound NAT same as another host that already has a Automatic static NAT or an inbound that is only for a specific port, I would add those above the Automatic NAT section as manual NAT rules.
Regards, Maarten