CustomValidator firing twice and not displaying error message

[Talisker]

I have a custom validator that checks to see if a checkbox is checked,
if it is then another field is required, so if it's blank then I want
to display an error. When I submit the form, for some reason it is
running the validator (server side) twice. It properly changes the
IsValid to False so the form doesn't get submitted, but the error
message doesn't display on the page.

Any idea why it would run twice (with nothing else running in between)
or not display the error?

Another thing to note, this is within the Edit template of a FormView
control in .net 2.0.

Any pointers would be welcome.