The 'DbProviderFactories' section can only appear once per config file

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

Locate section:
<system.data>
<DbProviderFactories>
<!-- providers added here -->
<add name="... />
</DbProviderFactories>

Line to be removed to resolve the issue
<DbProviderFactories/>
</system.data>
done!