Retry on duplicate key errors in Collection.Upsert
Retry on duplicate key errors in Collection.Upsert

Also, use IsDup to detect the duplicate key errors - it looks like
that's been accreted through painful experience.

DroiTaipei/mgo,Yottaa/mgo.v2,deafgoat/mgo,deafgoat/mgo,Yottaa/mgo.v2,hexadecy/mgo,Yottaa/mgo.v2,DroiTaipei/mgo,deafgoat/mgo,hexadecy/mgo,DroiTaipei/mgo,Yottaa/mgo.v2,hexadecy/mgo,deafgoat/mgo,DroiTaipei/mgo,hexadecy/mgo