BUG: fix bug that caused symbols to be added to the asset finder twice
BUG: fix bug that caused symbols to be added to the asset finder twice

alphaBenj/zipline,humdings/zipline,grundgruen/zipline,magne-max/zipline-ja,umuzungu/zipline,quantopian/zipline,enigmampc/catalyst,wilsonkichoi/zipline,alphaBenj/zipline,bartosh/zipline,nborggren/zipline,bartosh/zipline,wilsonkichoi/zipline,magne-max/zipline-ja,Scapogo/zipline,quantopian/zipline,grundgruen/zipline,florentchandelier/zipline,enigmampc/catalyst,Scapogo/zipline,nborggren/zipline,florentchandelier/zipline,umuzungu/zipline,humdings/zipline