Bug 26742

According to the official docs, all that needs to be added to the kernel
config file is "options NETGRAPH" ... however that is a sure way to get error messages on boot "module_register: module netgraph already exists!
linker_file_sysinit "netgraph.ko" failed to register! 17"

I found what looks like the solution to these messages at freebsddiary.org

"The above netgraph message can be eliminated by adding the following to
your kernel file, eliminating the need to load the netgraph ether module, which you will need for pppoe to function anyway.
  NETGRAPH_ETHER

In other words, these are the options that you will need to obviate the
need to load any modules.

options NETGRAPH
options NETGRAPH_ETHER
options NETGRAPH_PPPOE
options NETGRAPH_SOCKET


See extract from handbook .... as I read it only "options NETGRAPH"
is necessary, the other lines are "optional". Well I guess they are, but
it sure caused me some lost sleep until I found out was was going on !!

16.4.2. Kernel Configuration
You will need to set the following options in your kernel configuration file and then compile a new kernel.

  a.. options NETGRAPH

Optionally, you can add

  a.. options NETGRAPH_PPPOE

  b.. options NETGRAPH_SOCKET

although if this functionality is not available at runtime, ppp will load the relevant modules on demand

Fix: 

Alter handbook section "16.4.2. Kernel Configuration" to state ALL netgraph options should be included in kernel config file
How-To-Repeat: compile a kernel with ONLY "options NETGRAPH", not all relevant lines