The only thing that is discouraged in CASEing etiquette, would be copying another's original design for contest entry, publication, or other form of personal gain.
If you are duplicating a card for your own personal use, you are welcomed and encouraged to do so, and need not contact the original artist.
If you upload your duplicate to the internet, the original artist will always be highly flattered if you make note of their original as your source of inspiration, and/or, as Kathy said, you can provide viewers a link to the original work. It's like saying, "Thank you for inspiring me!" to the original artist.
Enjoy making your holiday cards! There are so many beautiful samples in the gallery sure to inspire!