Hello Bert2005
First of all, you dont have to waste another public IP address to achieve what you want, following will perfectly work for you
static (inside,outside) tcp 72.xx.xxx.100 3390 192.168.1.30 3389 netmask 255.255
.255.255 0 0
static (inside,outside) tcp 72.xx.xxx.100 3391 192.168.1.37 3389 netmask 255.255
.255.255 0 0
And about the error you get, you are not using a duplicate internal IP. You are using an external duplicate IP. 72.xx.xxx.99 is already one-to-one mapped to 1.100 and you cant re-map it to another internal IP, yet this approach is not necessary since you can use single public IP to achieve what you want by following the suggestion above
Regards