I already have the required letters from two teachers an a counselor, all that I feel will be fairly strong and show different aspects of me. Should I sent in an additional letter from an internship supervisor, or will this annoy admissions officers and just be repetitive?
Only send additional recommendation letters if they bring something new to the table. If it highlights certain traits that you didn't show in the classroom or elaborates on your passion for a certain subject, then do it. If you think that the internship advisor will have nothing new to say about you, it still wouldn't hurt to get the rec, but I suggest that you spend that time and energy on something else.
The letters that you already got are sufficient, but if you feel like , the person who you're gonna get this new letter from knows important aspects of you that you need to share, or if you feel like this person can put you in the best light... Go ahead and do it!
But if it's just a letter that you are adding, because you feel like showing more recommendation letters might make the person who reads this impressed... Don't do it.
Thats so funny because I am in a very similar situation: I would say it may have a downside but could also be very beneficial to your application- its up to you :)
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