duplicate.cs

public class Employee
{
    public int Id { get; set; }
    // ...
}

public class EmployeeComparer : IEqualityComparer<Employee>
{
    public bool Equals(Employee x, Employee y)
    {
        return x?.Id == y?.Id;
    }

    public int GetHashCode([DisallowNull] Employee obj)
    {
         return obj.Id.GetHashCode();
    }
} 

 static void Main(string[] args)
 {
       var department1 = new [] {
            new Employee { Id = 1 },
            new Employee { Id = 2 },
            new Employee { Id = 3 }
       };

       var department2 = new [] { 
            new Employee { Id = 3 },
            new Employee { Id = 4 }
        };
       
       // will find Employee Id = 3 only if comparer is in place
       var duplicates = department1.Intersect(department2, new EmployeeComparer());

      // could also use Contains
      // var duplicates = department1.Where(e => department2.Contains(e, new EmployeeComparer()));


       foreach(var duplicate in duplicates) {
            Console.WriteLine( duplicate.Id);
       }
}

linq.cs

var languages = new [] { "c#", "javascript", "python "};
var requirements = new [] { "go", "rust", "javascript" };

var union = requirements.Intersect(languages);

// will show: javascript
foreach(var result in union)
{
    Console.WriteLine(result);
}