Hi Guy,
First thanks for the beers! Much appreciated.
If this problem only happens in one site obviously it is not a problem in the code, it must be something about configuration or the mail server causing the duplicates. For example maybe you have mail forwarding setup and it is forwarded to multiple addresses that all land in the same inbox.
In any case 2.3.5.5 is a very old version of mojoPortal, about a year and a half has gone by since that release. I do not beleive it is a bug in the code but in general if you have problems I recommend to upgrade to the latest version. "If" you find a bug that is reproducable in the latest version I can fix it but I cannot fix old versions.
See Upgrading from Previous Versions and Important Skin changes before you upgrade.
In recent versions of mojoPortal the <system.net> section is no longer used so yo uno longer need to maintain those settings in Web.config, only the smtp settings in appSettings are used, see Email Configuration for details.
Hope that helps,
Joe