Someone illegally claimed me as a dependent, this has happened before in 2017 making it difficult for me. I do not know who it is I've reported in the past but what Now?

Someone has once again claimed me as a dependent in 2017 gthis happened and i had to go through alot just to get my taxes why are they allowed to do it again? Im 21 years old and take care of myself. I have no idea who is filing me or what i can do about it now. I also have my child as a dependent and it saus i must take her off because i am listed as a dependent on someone elses tax return.