It would seem that the story continue triggers twice. Once from _story.continue and another one in "DisplayNextLine();" which is just underneath.
My fix was to remove the "_story.Continue();" part.
https://github.com/KlaudiaBronowicka/VisualNovelTutorial/blob/1e53a63229176a28ba9753531d0a480c045a876a/Visual%20Novel%20Tutorial%20Part%202/Assets/Scripts/InkManager.cs#LL100C27-L100C27