Output below suggests me that upnp mappings are handled first, and thus win when the same port is mapped.
(And a portforwarding in its turn wins over manual dNAT rule)
sudo iptables -L -n -v -t nat Chain PREROUTING (policy ACCEPT 1264K packets, 168M bytes) pkts bytes target prot opt in out source destination 1487K 180M MINIUPNPD all -- * * 0.0.0.0/0 0.0.0.0/0 1487K 180M UBNT_PFOR_DNAT_HOOK all -- * * 0.0.0.0/0 0.0.0.0/0 1264K 168M VYATTA_PRE_DNAT_HOOK all -- * * 0.0.0.0/0 0.0.0.0/0
imho, manual mapping should take precedence over UPNP. Or upnp should figure out port is already in use