public interface IFoo
{
    bool Foo(Person a, Person b);
}

public class KungFoo : IFoo
{
    public bool Foo(Person a, Person b)
    {
        if (a.IsAmateur || b.IsAmateur) // common logic
          return true;
        return false;
    }
}

public class KongFoo : IFoo
{
    public bool Foo(Person a, Person b)
    {
        if (a.IsAmateur || b.IsAmateur) // common logic
          return false;
        return true;
    }
}

public interface IFoo
{
    bool Foo(Person a, Person b);
}

public abstract class FooBase : IFoo
{
    public bool Foo(Person a, Person b)
    {
        if (a.IsAmateur || b.IsAmateur) // common logic
          return true;
        return false;
    }
}

public class KungFoo : IFooBase
{

}

public class KongFoo : IFooBase
{
    public override bool Foo(Person a, Person b)
    {
        // Some other logic if the common logic doesn't work for you here
    }
}

public interface IFoo
{
    bool Foo(Person a, Person b);
}

public class KungFoo : FooImpl
{
    public override bool Foo(Person a, Person b)
    {
        if (this.IsAmateur(a, b))
            return true;
        return false;
    }
}

public class KongFoo : FooImpl
{
    public override bool Foo(Person a, Person b)
    {
        if (this.IsAmateur(a, b))
            return false;
        return true;
    }
}

public abstract class FooImpl : IFoo
{
    public abstract bool Foo(Person a, Person b);

    protected readonly Func<Person, Person, bool> IsAmateur = (a, b) => a.IsAmateur || b.IsAmateur;
}

public class Person
{
    public bool IsAmateur { get; set; }
}

public abstract class IFoo{
   bool Foo(Person a, Person b){
      if (a.IsAmateur || b.IsAmateur) // common logic
         return true;
   }
   public abstract Object otherFooMethod(Object o);
}

public class KungFoo : IFoo{
   //Foo already implemented

   public Object otherFooMethod(Object o){
      return o;
   }

}

public class KongFoo : IFoo
{
   public bool Foo(Person a, Person b)
   {
       if (a.IsAmateur || b.IsAmateur) // common logic
         return false;
       return !base.Foo();
   }

   public Object otherFooMethod(Object o){
      return o;
   }
 }