If I use, for example, as tags: tag1, tag2, tag1 tag2, is it considered duplicate content, or does it provide any benefit ?
What’s your keyword, if your keyword is 2 words, then use tag1 tag2. If your keyword is 1 word, then use tag1. I found 2 word keywords work good for me because there are hardly 1 word keyword that I can rank for, too general. So my suggestion to you is to use only tag1 tag2 rather than tag1, tag2, tag1 tag2.
the format of including keywords in web page is <meta name=“keywords” content=“”>. Here in side content you have to put your keywords. As spamers tried to include more and more keywords in to this in order to increase SERP results Google now a days giving less important to meta keywords. It is always good to select 3, 4 different keywords to each page and concentrate on this. Don’t repeat a keywords more than once while including in meta tags also do a proper keywords research before including keywords in to web page
Recently Matt Cutts advised to avoid using tags in wordpress post and therefore in first place I would suggest you to do same. However, if you want to use them use them carefully and don’t stuff them with your keywords.
I don’t think so that tags gonna help you anyhow . Their are lots of blogs using tags but yeah it helps any user to search your blogs entry.