I've made some research and the query does indeed come from Activity Monitor. Furthermore, the query is the same in SQL 2012 SP1. And for that matter, SQL 2014 CTP2.

I found a Connect item from 2009, https://connect.microsoft.com/SQLServer/feedback/details/496162/duplicated-lines-in-ssms-2008-r2-activity-monitor, that reported the problem. However, it was closed as Won't Fix. It was not a priority...