Warn and return early if Opal is already loaded
Warn and return early if Opal is already loaded

fixes #616
bbatsov/opal,bbatsov/opal,castwide/opal,jgaskins/opal,iliabylich/opal,Ajedi32/opal,jgaskins/opal,kachick/opal,suyesh/opal,jannishuebl/opal,castwide/opal,opal/opal,Ajedi32/opal,merongivian/opal,domgetter/opal,Mogztter/opal,merongivian/opal,c42engineering/opal,kachick/opal,iliabylich/opal,gabrielrios/opal,Flikofbluelight747/opal,gabrielrios/opal,merongivian/opal,Flikofbluelight747/opal,jannishuebl/opal,kachick/opal,gausie/opal,jgaskins/opal,Flikofbluelight747/opal,suyesh/opal,bbatsov/opal,Mogztter/opal,catprintlabs/opal,gabrielrios/opal,jannishuebl/opal,wied03/opal,iliabylich/opal,domgetter/opal,c42engineering/opal,gausie/opal,castwide/opal,opal/opal,fazibear/opal,Mogztter/opal,gausie/opal,kachick/opal,fazibear/opal,wied03/opal,Mogztter/opal,wied03/opal,opal/opal,catprintlabs/opal,fazibear/opal,catprintlabs/opal,c42engineering/opal,suyesh/opal,Ajedi32/opal,opal/opal