Let's say I edit a user's post and got approve for that, later (after got reputation for that edit) I realize that still there need to do some more edits in the same post, so Can I do modifications to the same post again? Is it a valid behavior?
Yes and no.
It's definitely better to make all edits at once. It is forbidden to edit twice on purpose. So try hard to avoid it as mods and other users can't see your purpose.
If you find post needs more changes, and your own edit was incomplete despite your best efforts to make it complete, then edit again. It's not good this happened, but second edit is better than leaving a post in need of reformatting without help.