This means you are using the mojoPortal package for .NET 3.5 but you are hosted under .NET 4. You should install the mojoPortal package for .NET 4.
Under .NET 4 some things that were previously required in Web.config (in .NET 3.5) are now in machine.config so if they exist in Web.config it causes this duplicate section error.
Hope it helps,
ps this question should have been posted in the Installation Forum not the developer forum