check for duplicate types in a list of emulation objects
check for duplicate types in a list of emulation objects

add Emulation.has_no_duplicates() static method that will raise an
exception if somebody tries to do something like
sudo tc qdisc add dev root netem delay 10ms delay 100ms. this is
technically acceptable but it can become really confusing in the end.
included is the EmulationHasDuplicatesError(exception) class to handle
the sad case in this method

serbant/simple_netem