Recently we did server migration and after which noticed the issue that few posts had the same post number.
Ran rebuild thread information with Rebuild position and post counters checked. Now the post number being same issue is resolved.
But there is another issue wherein when I start a new thread, it becomes the second post in the thread and the first post is from another thread,
Rebuild thread information seems to have fixed the post number issue in the earlier threads, but for all the new threads that is started post the rebuild, the issue seems to be persisting.