Is sharing of my Blog posts on social media by repeated readers bad for SEO on Google, Bing, Yahoo and DuckDuckGo as they probably will share posts on same profile or is it beneficial for SEO? Would like to read some Case studies if someone can find them.
According to my opinion social media sharing is helpful for boost the website traffic only. it will not helpful for keywords ranking.
Here is an good read on the subject: https://blog.kissmetrics.com/social-media-and-seo/ I don't believe it will hurt your efforts, but may not boost ranking much if at all either. Using them as free or paid traffic sources to your site is always worthwhile though!
Social sites like, Facebook, Twitter are not dofollow sites. To increase your brand awareness and to bring the traffic into your site you can utilize social media.