Hello Alexander,
I'm glad to hear that our networking options are expanding!
The 'lwip_tcpip_init()' should be called implicitly by the 'libc_lwip' library, so I believe your problems come from double initialization:
https://github.com/genodelabs/genode/blob/16.08/repos/libports/src/lib/libc_...
The 'lwip_libc' library will disappear in the near future, so to build against the current and upcoming release you would remove this dependency and let the the built-in socket support in libc take over. The TCP/IP stack would then be selected and configured at runtime via the VFS. For now you would be restricted to the Lxip stack but there is a LwIP plugin for the VFS that is nearly ready.
For an example of runtime network configuration see https://github.com/genodelabs/genode/blob/17.02/repos/libports/run/netty.run Note that you can use the VFS plugins internally or shared between components using the vfs server.
Cheers, Emery