Treat duplicate switch name in topology.conf as fatal error

One of our testers created an illegal topology.conf file.

He has a config you probably wouldn't see in production, but can see in
testing when you are sometimes given a collection of miscellaneous
resources.

          |-- nodes
switch1 --|
          |-- switch2 -- nodes

He tried the topology.conf file below. Switch s1 is defined twice. Slurm
accepted this config, but wouldn't allocate nodes from both switches to
one job.

SwitchName=s1 Nodes=xna[14-26]
SwitchName=s2 Nodes=xna[41-43]
SwitchName=s1 Switches=s2

I believe slurm shouldn't allow the second definition of switch s1. The
attached patch checks for duplicate switch names.
Patch from Rod Schultz, Bull.