M.P.
This is not good. Teach her how to express her feelings more directly. Stop her before she scratches herself. Say to her, I know you're mad, lets hit this pillow for example.
See if you can learn a different way of dealing with her so that she has more power and will be less likely to get that intensely angry. Give her choices. Do you want to pick up your toys now or in 5 minutes then set a timer for example.
Get the book, How to Talk so Children Will Listen and Listen so Kids will Talk. How we word things makes a really big difference in the way our kids respond to us.
I suggest that if you think in terms of teaching her instead of getting on her, your attitude and hence your way of wording and expectations for her behavior will be more positive and learning focused instead of punishment focused.
Yes, I have seen this in children and adults. It's an indication that the child/adult hasn't learned how to deal with anger, depression, feelings. When it's an adult cutting themselves it's a mental illness. When it's a young child, such as yours, it's more likely to be an indication that they need help in learning how to recognize and express their feelings. I suggest that it might be helpful to talk with her pediatrician about this and perhaps have her seen by a child psychologist. It might also help if you were to take a parenting class so that you can learn how to teach her about her feelings and how to positively express them.
I went back and skimmed your previous posts. You've been having difficulty with your daughter for some time now. I urge you to talk with her pediatrician and get a referral for a professional evaluation. I also suggest that you'll benefit from parenting classes as well as reading some parenting books. I recommend Love and Logic books by Foster Cline. For effective discipline techniques I suggest 1-2-3 Magic.
I suggest that it's possible you've been able to stop or reduce the screaming which has now resulted in the scratching. Both are ways to express emotion. We need to teach appropriate behavior without trying to stop the feelings.
It helps to recognize your child's feelings while showing her how to deal with them. "I see you're angry. It's OK to be angry. It's not OK to scratch yourself." I've often had my daughter and now my grandchildren scream with me saying, "I'm so angry!" A few minutes of loud screaming gets rid of the energy that anger produces and allows everyone to calm down.
Another way to deal with anger is to put them in their room or another quiet place and give them time to calm down. My grandchildren go to their room and play until they're able to come out and be more compliant. It's not punishment. It's discipline, teaching them how to calm themselves. I would stop the scratching while talking with her soothingly before sending her to a quiet place to get control of herself.