My webiste was getting good traffic but after the one duplicate post, the entire website is not getting any traffic. I can see many urls are removed in search console. What to do next? Please help.
As written in the above comment just a duplicate post can't cause this to happen.
Quality SEO work with even one-page duplication won't impact as you are indicating.
I would encourage you to think and analyse what you might have done over last few weeks/months.
And if you may have done any implementation which doesn't follow the Google webmaster guidelines then you need to do some cleanup.
Also, mostly this kind of things happens when you do poor quality link building.
So, i would recommend you to analyse your SEO work pattern and do require changes.