i got the solution for this problem.
this is just because you have the same parameter value in GET while getting the user object for filling the form and in POST for submitting changes. formbackingObject is called twice, once when the form is displayd and once the form is submitted. If submitting, the formBackingObject reads the username-param from POST AND GET and seperates both with a COMMA.
just rename the parameter in the url and have fun!