The procedure WritePage was called twice for template &1.
&1 - The template specified on the second call of the WritePage API.
During the handling of one request, the WritePage function has been called twice - this means that the function tries to send two HTML pages to the browser simultaneously. The browser can't handle this.
Check the handling of requests in your programs - find the request, that can lead to two calls to the WritePage API - change the logic so that only one WritePage call can be made for each request.