Heatmap of compound curse words used online. (sourced from reddit: 2006-2020)

As an out of touch person, I have a possible explanation for this: have you ever said "shit" repeatedly as something goes wrong? I imagine some people would write a story where that happens and write it as "shitshitshit" and not "shit shit shit". But outside of that situation I have never seen or heard "shitshit".