combination of two field must be unique in Entity Framework code first approach. how it would?

combination of two field must be unique in Entity Framework code first
approach. how it would?

I have two class contacts and groups
Combination of FirstName and LastName must be unique and can add multiple
addresses for a single contact. How I can done this in entity framework
code first approach?
public class Contacts
{
[Key]
public int ContactID { get; set; }
[ForeignKey("Group")]
public int GroupID { get; set; }
[Required]
public string FirstName { get; set; }
[Required]
public string LastName { get; set; }
[Required]
public string Address { get; set; }
[Required]
public string Number { get; set; }
[Required]
[EmailAddress]
public string EmailId { get; set; }
[DataType(DataType.Date)]
public DateTime CreateDate { get; set; }
public DateTime? ModifiedDate { get; set; }
public virtual Groups Group { get; set; }
}
public class Groups
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int GroupID { get; set; }
[Required]
public string GroupName { get; set; }
[Required]
public string GroupDiscription { get; set; }
public DateTime CreateDate { get; set; }
public DateTime ModifiedDate { get; set; }
}