Recently, Instagram allowed users to share the story of other users. From our understanding, this is only available for those that are public. If you are watching a story of a private Instagram user you cannot send their story to another person in which they are not friends. We will confirm this. All that being said, there is not a notification to let you know when someone shares your story to someone else.
If this is a problem for you, you may want to reconsider sharing your stories to the public. You can always make your Instagram account private. There may also be an update that allows you to block people from sharing your Instagram Story as well. As we learn more, we will update this article.
Have you tested this? When you share a story, what does Instagram say?