warn about duplicate constant only if reference is already in registry
warn about duplicate constant only if reference is already in registry

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