It is very common nowadays, specially with smartphone videos and photos. It consists of adding the same picture zoomed in the background, to turn a portrait image into a landscape one, as seen below. I'm sure it's super obvious but I've googled it without success.
Banding the sides of an image is called pillarboxing (as opposed to banding the top and bottom, i.e., letterboxing). Wikipedia's page refers to the technique you describe as "stylized pillarboxing" or "echo pillarboxing".