modelBuilder.Entity<Book>()
            .HasMany<UserData>(s => s.user)
            .WithMany(c => c.book)
            .Map(cs =>
            {
                cs.MapLeftKey("bookID");
                cs.MapRightKey("userID");
                cs.ToTable("BookXUser");
            });

        modelBuilder.Entity<BookXUser>()
            .HasRequired<UserData>(s => s.user)
            .WithMany(g => g.bookXuser)
            .HasForeignKey<int>(s => s.userID);

        modelBuilder.Entity<BookXUser>()
            .HasRequired<UserData>(s => s.user)
            .WithMany(g => g.bookXuser)
            .HasForeignKey<int>(s => s.userID);

The EntitySet 'BookUserData' with schema 'dbo' and table 'BookXUser' was already defined. Each EntitySet must refer to a unique schema and table.

public class BookXUser : IEntityBase
{
    public int ID { get; set; }
    public int bookID { get; set; }
    public virtual Book book { get; set; }
    public int userID { get; set; }
    public virtual UserData user { get; set; }
    public string bookShelf { get; set; }
}