Test that receivers are only called once for each emit.
Test that receivers are only called once for each emit.

See #109.

alecthomas/entityx,Bablawn3d5/entityx,jaynus/entityx,alecthomas/entityx,jarrettchisholm/entityx,jaynus/entityx,jarrettchisholm/entityx,jaynus/entityx,Bablawn3d5/entityx,arvidsson/entityx,arvidsson/entityx,Bablawn3d5/entityx,alecthomas/entityx,arvidsson/entityx,jarrettchisholm/entityx