title: system.data column 'invariantname' is constrained to be unique
body: <p>I'm using FirebirdSqlProvider 4.8.1.1 and EntityFramework 6.0
  I get the exception <code>system.data column 'invariantname' is constrained to be unique</code> when I'm trying to use EntityFramework.
  My <code>app.config</code> file:</p>
  
  <pre><code>&lt;entityFramework&gt;
      &lt;defaultConnectionFactory type="FirebirdSql.Data.EntityFramework6.FbConnectionFactory, EntityFramework.Firebird" /&gt;
      &lt;providers&gt;
        &lt;provider invariantName="FirebirdSql.Data.FirebirdClient" type="FirebirdSql.Data.EntityFramework6.FbProviderServices, EntityFramework.Firebird" /&gt;
        &lt;provider invariantName="System.Data.SqlClient" type="System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer" /&gt;
      &lt;/providers&gt;
  &lt;/entityFramework&gt;
  &lt;system.data&gt;
      &lt;DbProviderFactories&gt;
        &lt;remove invariant="FirebirdSql.Data.FirebirdClient" /&gt;
        &lt;add name="FirebirdClient Data Provider" invariant="FirebirdSql.Data.FirebirdClient" description=".NET Framework Data Provider for Firebird" type="FirebirdSql.Data.FirebirdClient.FirebirdClientFactory, FirebirdSql.Data.FirebirdClient" /&gt;
      &lt;/DbProviderFactories&gt;
  &lt;/system.data&gt;
  </code></pre>
  
  <p>I have only one DbProviderFactories entity in my <code>machine.confgi</code> file.
  How to fix it? </p>