Please note that you can only have 1 site with weebly per account.

If you need to add multiple addon domains to cpanel on a main account running a weebly site, please make sure you do not add them to the public_html directory of the main account.
so instead of using /home/userid/public_html/addondomainname you best create the addon domains in their own directory structure i.e. /home/userid/addondomainname