Insert Record in Many to Many relationship using generic repository

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repository

entity-framework

I have many to many relationship between doctor and clinic entities and also i am using unit of work generic Repository pattern. When i try to insert new doctor record i need to select multiple or single existing clinics. How can i insert doctor record with clinic reference and without duplication error.

DoctorManager.cs
****************

public void InsertDoctor(Doctor doctor)
{
   //doctor object has clinics selected
   _unitOfWork.DoctorRepository.Insert(doctor);
   _unitOfWork.Save();
}

GenericRepository.cs

public virtual void Insert(TEntity entity)
{
   _dbSet.Add(entity);
}

How can i implement _dbSet.Attach(); in generic repository to ovoid duplication error.
Could someone direct me with sample would be thankful.

namespace DoctorBooking.DomainEntities
{
   public class Doctor
   {
     [Key]
     public Int32 DoctorId { get; set; }
     public string LastName { get; set; }
     public string FirstName { get; set; }

     public virtual ICollection Clinics { get; set; }

    }
}

namespace DoctorBooking.DomainEntities
{
   public class Clinic
   {
     [Key]
     public Int32 ClinicId { get; set; }
     public string Name { get; set; }
     public int ContactNumber { get; set; }
     public string Email { get; set; }

     public virtual ICollection Doctors { get; set; }

   }
}