Hi! I started my master's earlier this year and I have a great advisor. My co-advisor, on the other hand, even though she's great and helps me a great deal, sometimes I think she helps me too much. She constantly performs the experiments without me knowing and then tells my advisor about the results and sometimes when I'm doing them, she tries to take control of the process, by telling me how to do it - her way, not necessarily the best or the right way - or she just sits on my place and starts doing things herself. It feels like my project is hers and I'm just an intern or someone who does the experiments instead of a master's student. Every Monday we have a lab meeting and yesterday was my turn to present my results. When people asked me questions, she talked over me and once she even talked over my advisor. People noticed and now I know it's not just a product of my imagination. And I really don't know how to deal with it because when I actually need her help, she's great. I just feel like she doesn't know her limits and I don't know how to set them without hurting her feelings.
Any ideas? Has anyone ever been through something similar?

That sounds really awkward and frustrating. I haven't been in a similar position and this sounds like a difficult situation to resolve without hurting anyone's feelings. I think a polite conversation with your main supervisor might be best. or if that is too confrontational it might be better to solve the problem in little steps and gentle nudges to get her to change.

I would focus on trying to get her into the teacher role. Like say that you want to practice presentations for some upcoming conference and that you want to answer difficult questions yourself. Or that you feel that you can't include data in your thesis if you didn't do it yourself. Therefore her doing experiments is harming you or duplicating results. Also you could say that your are doing a specific methodology even though it might be wrong because you want to learn it/ improve at it. She clearly wants to help you but if you are firm about what you want to do she will support you.

Hey, thank you. That's a very reasonable solution. I'll try and work it out with her and if it doesn't work, then I'll speak to my main supervisor and hope for the best. Thanks again!

I had this problem too. My solution was to stop asking that person for help because they would have a solution on my desk by morning.
I kept my progress to myself, under-reported what I was doing and kept communications to a minimum. Any questions were kept vague and I stopped revealing my plans for the week or month ahead. I also talked to others in their presence about unrelated work to send a message that I had other issues. The co-supervisor lost interest pretty quickly.

Try to figure out why the co-sup is doing this. Boredom with their own work? They see a chance of getting a quick paper? They use you as a chance to deflect from difficulties in their own work? Knowing this will help you deal with the problem.