The 'DbProviderFactories' section can only appear once per config file" ERROR message

This happens when the section is duplicated in the machine.config file. This is a known .NET issue. To fix it, remove the duplicate from the machine.config files.

Two files to modify:

C:\Windows\Microsoft.NET\Framework\v4.0.30319\Config\machine.config

C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Config\machine.config