Sadly this happens ocassionally here but not as often as you may think. Musing use manual curation and Im sure dont have the time to run every answer through a plagiarism checker.
The quality here is very good and there has to be some trust!
If you come across plagiarism on Steemit in general, repor it to @steemcleaners . They are extremely active in trying to flag out scammers, spammers and p!agiarists.
Id also call them out on it and post a comment under theirs telling them to stop cheating
Hope this helps and kudos to you for flagging it up :-)