My [22M] Coworker [27F] is doing my job without my permission. How can I tell her to stop without being hostile?

I work in a new hospital cafe doing Utility/Inventory. The cafe has been open since July. I have been working there since August. I am the only employee on the team who is part-time. I work Monday, Wednesday, and Friday from 8:00am-4:30pm. My duties and responsibilities are washing dishes, cleaning the dining area, outside courtyard, kitchen, and sales floor. I also do inventory counts, grabbing new deliveries, labeling and throwing out old items, etc.

On the days I am not there, a few of my coworkers wash dishes and clean the dining areas and courtyard.

On the days when I am there, if I am on my break or if I am doing inventory, cleaning, etc. I don't mind my coworkers in the back of the kitchen to help me wash dishes. Even when I am washing dishes, if they are helping me put the dishes away or rinsing them out and handing them to me, I am okay with that. What I don't like is when they take over washing dishes when I didn't give them permission to do that.

My new coworker that just started does this. Don't get me wrong, she is a good worker but her doing this is a bit irritating.

As I am washing dishes, she'll rinse all the dishes out and throw away the food for me and put the dishes away after I am done cleaning them. However, once I step away for a second to put the dry dishes away and transfer the dishes over from one sink to another, she starts taking over. It's annoying because I didn't tell her that she can take over.

I would rather wash dishes myself because it is MY job. I appreciate the help, but I don't want her completely taking over my duty. I don't mind her rinsing the dishes out and handing them to me or helping me put them away. However, I don't like her taking over.

I don't know how to tell her to stop doing this without being hostile. What do you guys say?

tl;dr: My coworker has been taking over my job without me giving her permission to do so. I understand she is trying to help, but it's a little irritating. My job is MY job. How do I tell her to stop without coming off as an a-hole?

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