It is somewhat normal in the sense that a lot of teenagers do it but it teaches a bad lesson so I would also say it's horrifically toxic. I'm in my mid 20's (M26) now but, when I was a teenager, me and my then girlfriend at the time shared each other's logins but she spent all day checking mine. If any girl messaged me or anything aggravating came up - my toxic girlfriend would ALWAYS coincidently happen to have just innocently got to her computer and I "must have left myself logged in because the message just popped up." I naively believed several times but it became clear she was in there 24/7 it emotionally scarred me for a long time through my next relationships and I hated having phones or being on socials because I thought it would cause drama to have any friend requests pop up and cause arguments I used to close myself off and decline any colleagues or new school friends because I was essentially taught getting a new follower would cause an argument. Turns out that girl was cheating on me a lot so I think it's her own guilt she would check mine. So to conclude, yes it's normal for teenagers to do that but it's horrifically toxic!