add flag to ensure the callback is __never__ invoked twice
add flag to ensure the callback is __never__ invoked twice

targos/superagent,noikiy/suberagent,deltreey/superagent,KingMario/superagent,ccapndave/superagent,bevacqua/superagent,thejameskyle/superagent,RubenVerborgh/superagent,wushuyi/superagent,eligolding/superagent,jarvisaoieong/superagent,sutoiku/superagent,philipshurpik/superagent,ertrzyiks/superagent,stasha/superagent,yussan/superagent,simonvizzini/superagent,classdojo/superagent,focusaurus/superagent,h1bomb/superagent,modulexcite/superagent,kmalakoff/superagent-ls,aaroncrows/superagent,alinz/superagent,killerlayup/superagent,jarvisaoieong/superagent,KuroGuo/suberagent,andyburke/superagent,nemisj/superagent,thejameskyle/superagent,sutoiku/superagent,sebastianvera/superagent,soyuka/superagent,drinks/superagent,nemisj/superagent,targos/superagent,kmalakoff/superagent-ls,stanzi83/superagent,brainbytes/superagent,stasha/superagent,ccapndave/superagent,classdojo/superagent,philipshurpik/superagent,visionmedia/superagent,RubenVerborgh/superagent,KuroGuo/suberagent,visionmedia/superagent,geminiyellow/superagent,craigspaeth/superagent,yeatszhang/superagent,sebastianvera/superagent,mvanlonden/superagent-webpack-client,aaroncrows/superagent,reinaldo13/superagent,craigspaeth/superagent,brotchie/superagent,yeatszhang/superagent,brotchie/superagent,puterakahfi/superagent,focusaurus/superagent,andyburke/superagent,ertrzyiks/superagent,deltreey/superagent,zsh2938/superagent,alinz/superagent,yussan/superagent,eligolding/superagent,noikiy/suberagent,fredigato/superagent,stanzi83/superagent,zsh2938/superagent,drinks/superagent,killerlayup/superagent,geminiyellow/superagent,puterakahfi/superagent,h1bomb/superagent,simonvizzini/superagent,soyuka/superagent,modulexcite/superagent,wushuyi/superagent,KingMario/superagent,brainbytes/superagent,mvanlonden/superagent-webpack-client,fredigato/superagent,visionmedia/superagent,reinaldo13/superagent