Should you write XML comments for Interfaces, concrete implementations, or both?

I am interested in where I should apply my XML comments. Should I put a more generic XML comment in the interface and a more descriptive one on the implementing class? Like this:

public interface IObjectRepository
{
    /// <summary>
    ///    Returns an object from the respository that contains the specified ID.
    /// </summary>
    Object GetObject(int Id);
}

public ObjectRepository : IObjectRepository
{
    /// <summary>
    ///    Retrieves an object from the database that contains the specified ID.
    /// </summary>
    public Object GetObject(int Id)
    {
        Object myData = // Get from DB code.
        return myData;
    }
}

I did not include <param> for simplicity's sake.

Is that a good practice for comments or is there a different way? Do I just skip commenting the interface?