How to fix Column 'InvariantName' is constrained to be unique

Introduction 

When you build a web project that uses Enterprise Library Community for the Application Data Block and the MySQL .NET / Connector, VisualStudio throws the error  "Column 'InvariantName' is constrained to be unique. Value 'MySql.Data.MySqlClient' is already present", but you added it only once. 

Is very simple to fix, you need to add "<remove invariant="MySql.Data.MySqlClient"></remove>" to the start of system.data > DbProviderFactories node. 

 Background

You can be confused if you used XML transform for Web.Config, Web.Debug.Config or Web.Release.Config.

But you shouldn't focus in that. The problem is the provider's installation put in machine.config the childNode <add name="MySQL Data Provider" invariant="MySql.Data.MySqlClient" and normally you put the same manually in your web.config file. So at the compiling time, when web.config heritates maching.config, it results in duplicate "add" child with the same invariant attribute, with the value "MySql.Data.MySqlClient" .

The examples of use around Internet and the tutorials of ELContrib Application Data Block add the DbProviderFactories node to System.Data element. It suposes that the MySQL .NET/Connector aren't installed in system. It suposes the mysql.data.dll is referenced in any directory and the configuration isn't heritated from machine.config. 

Using the code 

Your XML block must be like this to solve the problem: 

<system.data>
  <DbProviderFactories>
    <remove invariant="MySql.Data.MySqlClient"></remove>
    <add name="MySQL Data Provider" invariant="MySql.Data.MySqlClient" description=".Net Framework Data Provider for MySQL" type="MySql.Data.MySqlClient.MySqlClientFactory, MySql.Data, Version=6.2.4.0, Culture=neutral" />
  </DbProviderFactories>
</system.data>

Remember to add <remove invariant="MySql.Data.MySqlClient"></remove> at the start of DbProviderFactories.