Existing work has studied a related albeit different task, novel document detection (Kasiviswanathan et al., 2012(Kasiviswanathan et al., , 2013Zhang et al., 2002Zhang et al., , 2004, where one aims to identify from a document stream if a newly arriving document is novel or redundant.
this task assumes all the previous documents are known to be "normal", and only checks if a new document is novel.