There's no strong evidence here (that I can see at a glance) that this person merely copied your post and summarised your key points. Naturally, if there's not much to say and different people share a similar point of view to you, your answers are going to end up saying similar stuff, and the mere fact they did isn't an indicator they did anything wrong.
Even if they did, though, this is fine, for a few reasons:
1. People can use your stuff. That's part of the point.
Your contributions on this site are licensed under CC-BY-SA. This means people can use, change and redistribute your stuff so long as they share under the same license and attribute your work.
However, if they completely rewrite everything to the point the original is not even recognisable, they aren't using your work, and don't need to provide any attribution at all. And supposing they did use your work, it's not recognisable anymore.
2. New posts expressing the same idea are welcome if they're better.
Suppose someone comes along and states a few points very poorly, in a way that's hard to understand or not as well written as it could be. Other answers do similarly.
If a new answer arrives which provides the same information, in a way that's easy to understand, thoughtfully explained, complete with examples and pictures (if necessary), that's immensely more useful than its predecessors. This improves the quality of the site and is completely welcome.
Overall, community moderation will take care of it.
If someone provides no new information in their answer, generally people respond to that by downvoting, and often leave comments to the effect of "this adds nothing new over what X said".
If they provide something new and useful (as in #2), they've earned the votes they receive.
Overall, if two posts are explaining similar stuff, the one that is most useful is going to receive more positive feedback and votes. You might notice yours has received more positive feedback and votes and is in a higher placement.
Some users sort answers by age, rather than by vote count, so gradually your crafted answer goes down. Should i flag their answer?
This has nothing to do with this person theoretically using your answer and is a natural consequence of new answers arriving. Live with it. That's the point of the Active view. People with the Oldest or Votes view will not have your answer placement affected merely by new answers.