Forms authentication duplicating ReturnURL

[AC]

Hi

We have forms authentication setup to redirect to 'logonrequest.aspx'.
This works fine for the majority of the time but on occasion the
ReturnUL parameter of the querystring gets duplicated (once as it
should be and the second time it gets encoded). This causes the
browser to become confused and try to redirect to an unknown page
which causes a 403 forbidden error.

Does any body know why the return url should get duplicated like this.
Its really annoying as it only happens sometimes, usually after
postbacks (i.e. using a form a few times with postback then waiting
for a the session to timeout before getting the error. Doesnt seem to
happen if the form has not been posted back.

Thanks in advance.

AC