This happens to me when I have two fields with the same name, such as

<INPUT TYPE="text" NAME="userid" VALUE="$rec{'userid'}">
<INPUT TYPE="hidden" NAME="userid" VALUE="$rec{'userid'}">

On your page click "view source" on your web browser and very carefully check all of your input fields and see if you have two with the same name. It usually occurs to me when (like above) I have a hidden field and a text field with the same names.

Good Luck!