[Zope] surprising interaction between Javascript and Zope

[Dennis Allison]

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.