If you enable a dynamic target in which the source is a query string in step N and the target is the same query string in step N+1, the load test duplicates the query string like this:
...Timesheets/TITimesheet.aspx?TimesheetID=?TimesheetID=423%22
The first "TimesheetID=" with no value should not be present. The second one is correct.
Also attached is a Fiddler trace of the load test running on my machine (build 2012.2.1527) which shows the problem. Look at the query strings in the URL for session 7.