With this thought in mind, the next time this person tries to interfere or undermine you in any way at all, I would recommend pulling them aside for a quiet but firm conversation. Say something like: I’ve noticed that you seem to enjoy critiquing my work. I am sure you mean well, but I would really prefer if you wouldn’t do it as I am getting sufficient feedback from my line manager. I promise to ask if I need your help.

This conversation should flush out a response from them that explains why they have been acting in such an annoying way towards you. As you’ve spelt out that they are not your line manager, this should at least embarrass them into some sort of explanation and they’ll hopefully offer you a fulsome apology. If they protest about “just wanting to help”, then reiterate that you already have all of the guidance you need from those who are senior to you. This way, you are getting the message across that you two are in fact of equal status.

Honest Boss: “Someone with the same job description as me keeps acting like my boss”