For example:
Some ip/port -> router -> computer 1 (intended destination) & computer 2
I want the router to "packet forward" from an ip or a port to not just its destination but to also another computer on the network.
Also computer 1 and computer 2 shouldnt know that the packet has also been sent somewhere else.
This will only work with UDP (or other connectionless protocols). You can use multicast, and make computer1 and computer2 join a multicast group, however this will only work if the interim routers support multicast routing. You simply send to the multicast address from the sender machine.
The other option is some iptables black magic on the router, see here:
http://www.bjou.de/blog/2008/05/howto-copyteeclone-network-traffic-using-iptables/
