I have a blog installed on my site, many people are posting comment on my post
I have checked these comments are already available on many sites, people are posting same comments on many sites
I want to know these comment worth for my site or I have to delete it
looking for the suggestion from experts
Thanks
I dont think duplicate comment will effect your site as it is posted from different ip address. But it good to keep new comments.
Just delete the comments that you think is spam. Check the site of the commenter and see if the site is quality if not you can also delete that comment.
Most likely those are spaming posts for backlinks. I ahev disable comment postings on my blog. So, it is upto you to decide if you want others to post their links on your website.
Spam comments is one of the thing you will have to deal with as a blog owner. It is a problem that most people know too well about. Like @webspider said above, leaving those comments make your site look popular. You might consider making your comment available only to those who sign in.
If the comments are relevant and are not spamming you can leave them, because sometimes some comments shield more light on a post and even update a post.
I take out comments without any relevance to my post whatsoever. They just look like clutter. I keep those that are sensible, regardless of whether they are just dropping links. I have no problem making my blog dofollow, because I know how it is to be at their position. I have done the same thing, too. So nothing wrong with helping one another 
And hey, there really are people paid to comment on blogs and drop links but really read them and learn a thing or two. So if my primary purpose of keeping a blog is to inform and educate people, then I don’t think comments with links does not in any way interfere with this objective.
Everyone try to get backlinks in many ways and in my view blog commenting now a days everyone keep on posting something and give a url thinking they would get a backlink. But it does help only at few point. If the post is really valid you can add it no matter what ? give the link as no-follow. But if it is some kind of promotion then just spam it.
Good strategy is to have all your comments as dofollow link - that way you will attract whole bunch of visitors (people trying to leave backlink on comments), plus your blog will be linked from other blogs in those “dofollow blog lists”.
But dont forget to use moderation approval, and let go only comments that you consider to be valuable add to your blog post.
Spammy comment may be too much back links with keywords you may modify them but should not delete them because more and more comments on your blog posts make them popular on search engines and that thing will helps to improve blog popularity and search engines likes those kind of blogs and SE’s automatically promote them.
if the comment doesnt relevant or looks like really a spam then delete it…moderating comments would be a good idea
Put a authentication on comments area, use nofollow tag for any link used in the comment and use moderation approval if necessary, it will slow down the spam comments.
If the comments are valid and good then keep them. Multiple dupicate comments from the same person do not make your blog look good and if anything badly moderated. If unsure abotu a comment just remove it - better that way.
Our blog gets comments that are outright spam and then those that can be considered borderline. The borderline ones often talk about something in the article. For example, they might say “Great point” and mention why they agree or provide their own suggestions. However, those close out the comment with their name and a link to their blog or site.
These I usually leave up because while they are link dropping, they also took to the time to actually skim the blog post and leave a somewhat relevant comment.
Anything else I delete though and while having a lot of comments on your blog will make it appear popular, I won’t personally want it overrun with spam.
Many people comment on blogs just to have backlinks.
To keep your blog at a good quality just remove the spammy comments or make them nofollow.
When people post and the comment looks the same then either delete it or modify it and take out all links that way your site looks more populr.
Might make your blog appear popular
If visitor post comment on your blog post with unique content so its beneficial but visitor post duplicate comments so you can remove your self in your blog.
Well these comments will not harm your site but they all comments are spam and they are using in all sites.
There will be no effect through these comments but if you get an excessive comments like spamming you can delete it and make it nofollow.