[Zope] surprising interaction between Javascript and Zope

We have a need to set a session variable concurrently with a user submitting a form on a mouse click. I accomplish this with a bit of JavaScript associated with the OnClick attribute of the form: location.replace('/setSesVar?var=nams&val=xxx;frm.submit(); which does the job except that the setSesVar script is executed twice, a fact we discovered by monitoring calls to the setSesVar script. Once with an incorrect parameter, and secondly with a correct parameter. There is probably some obvious explanation, but I am stumped. Setting invalid values into session variables, even transient invalid values, can cause problems.