overriding abstract methods in an inherited abstract class

Just omit the declaration of execute() in Binary at all. Since Binary is abstract as well, you don't have to implement any abstract methods of its ancestors.

You don't need to declare execute() in the Binary class since it's already inherited from Command. Abstract methods don't need to be implemented by other abstract classes - the requirement is passed on to the eventual concrete classes.

public abstract class Command
{
    public abstract object execute();
}

public abstract class Binary : Command
{
    //the execute object is inherited from the command class.
}

public class Multiply : Binary
{
    public override object execute()
    {
        //do stuff
    }
}