title: Trouble when trying to create a database with Entity Framework and migrations
body: <p>I want to create database EF migrations via the developer command prompt for VS2015. When I try to use this command line:</p>
  
  <pre><code>dotnet ef migrations add v1
  </code></pre>
  
  <p>I get this error:</p>
  
  <blockquote>
    <p>The property 'partCategoriess' cannot be added to the entity type
    'PartCategoryPart' because a navigation property with the same name
    already exists on entity type 'PartCategoryPart'.</p>
  </blockquote>
  
  <p>Is anything wrong with the <code>DbContext</code>? I am trying to create a many-to-many table between <code>categoryParts</code> and <code>parts</code>.</p>
  
  <pre><code>public class ShoppingDbContext : IdentityDbContext&lt;User&gt;
  {
      public ShoppingDbContext(DbContextOptions options) : base(options)
      {
      }
  
      protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
      {
          base.OnConfiguring(optionsBuilder);
      }
  
      public DbSet&lt;PartCategory&gt; PartCategories { get; set; }
      public DbSet&lt;Part&gt; Parts { get; set; }
  
      protected override void OnModelCreating(ModelBuilder modelBuilder)
      {
          base.OnModelCreating(modelBuilder);            
  
          modelBuilder.Entity&lt;PartCategoryPart&gt;()
              .HasKey(t =&gt; new { t.partCategoriess, t.Part });
  
          modelBuilder.Entity&lt;PartCategoryPart&gt;()
              .HasOne(pt =&gt; pt.partCategoriess)
              .WithMany(p =&gt; p.PartCategoryPart)
              .HasForeignKey(pt =&gt; pt.PartCategoryId);
  
          modelBuilder.Entity&lt;PartCategoryPart&gt;()
              .HasOne(pt =&gt; pt.Part)
              .WithMany(t =&gt; t.PartCategoryPart)
              .HasForeignKey(pt =&gt; pt.PartId);
      }
  }
  
  public class PartCategoryPart
  {
      public int Id { get; set; }
  
      public int PartCategoryId { get; set; }
      public PartCategory partCategoriess { get; set; }
  
      public int PartId { get; set; }
      public Part Part { get; set; }        
  }
  
  public class PartCategory
  {
      public int PartCategoryId { get; set; }
      public string Category { get; set; }
      public List&lt;ProductPartCategory&gt; ProductPartCategories { get; set; }
  
      public List&lt;PartCategoryPart&gt; PartCategoryPart { get; set; }
  }
  
  public class Part 
  {
      public int PartId { get; set; }
      public string Code { get; set; }       
      public string Name { get; set; }
      public double? Price { get; set; }
      public List&lt;PartCategoryPart&gt; PartCategoryPart { get; set; }
  }
  </code></pre>