Lets say I got a question open in my browser and started preparing answer for it. When I came back in the browser and posted my answer, I found the same answer was already given by someone else. Since I didn't refresh the page I didn't get that someone has posted the answer. So what can I do? Should I keep the answer though its almost similar? Or I should delete my answer?
We can't tell you what you should do. We have no way to know how users will react to duplicate answers.
This question has been addressed on the main meta (the core concepts here are really not anything specific about this site):