MEF DirectoryCatalog reads the same dll many times

RegistrationBuilder regisBuilder = new RegistrationBuilder();
regisBuilder.ForTypesDerivedFrom(typeof(IDCPlugin)).Export<IDCPlugin>();

var catalog = new DirectoryCatalog(AppDomain.CurrentDomain.BaseDirectory+@"\Parts", regisBuilder);
var agcatalogue = new AggregateCatalog(catalog);
var container = new CompositionContainer(agcatalogue, CompositionOptions.DisableSilentRejection);
container.ComposeParts();

var v = container.GetExportedValues<IDCPlugin>();
foreach (var typeInstance in v)
{
    Console.WriteLine(typeInstance.Identifier+":"+typeInstance.Description);
}

// At this point I have one class imported more times* even if I copy different assemblies into the directory (same interface).
// *more times = number of assemblies there are in the directory.