add test to check that multiple equivalent keys in differentials dict will fail [docs only]
add test to check that multiple equivalent keys in differentials dict will fail [docs only]

dhomeier/astropy,mhvk/astropy,saimn/astropy,dhomeier/astropy,mhvk/astropy,funbaker/astropy,MSeifert04/astropy,lpsinger/astropy,pllim/astropy,dhomeier/astropy,larrybradley/astropy,AustereCuriosity/astropy,AustereCuriosity/astropy,DougBurke/astropy,astropy/astropy,saimn/astropy,pllim/astropy,MSeifert04/astropy,pllim/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,MSeifert04/astropy,dhomeier/astropy,kelle/astropy,larrybradley/astropy,saimn/astropy,saimn/astropy,DougBurke/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,funbaker/astropy,stargaser/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,kelle/astropy,lpsinger/astropy,mhvk/astropy,DougBurke/astropy,bsipocz/astropy,astropy/astropy,bsipocz/astropy,lpsinger/astropy,stargaser/astropy,AustereCuriosity/astropy,StuartLittlefair/astropy,mhvk/astropy,kelle/astropy,larrybradley/astropy,funbaker/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,DougBurke/astropy,kelle/astropy,mhvk/astropy,pllim/astropy,astropy/astropy,kelle/astropy,bsipocz/astropy,AustereCuriosity/astropy,larrybradley/astropy,astropy/astropy,saimn/astropy,stargaser/astropy,AustereCuriosity/astropy,pllim/astropy,MSeifert04/astropy,lpsinger/astropy,StuartLittlefair/astropy,stargaser/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,funbaker/astropy,astropy/astropy,bsipocz/astropy