If your applications gets big , you may encounter this error "ERR-3331 This page was already submitted and can not be re-submitted while importing your application." while importing into APEX.
This is caused by the OHS (Oracle HTTP Server / Apache ) timing out.
Even if you get this error, the Oracle process that imports your application will continue until it ends successfully, so your application will be imported correctly.
To avoid having this error, simply change the OHS configuration. In the httpd.conf file located in your Apache/conf directory and increase the Timeout parameter.
# Timeout: The number of seconds before receives and sends time out.
You can find more information in this thread in the APEX forum.