Is it polite to "plagiarize" content that you contributed to others' answers?

Question

I could have posted this question earlier but thanks to by bad memory I can finally ask it.

Here's my answer and a competing answer under the same question. At first glance, the first paragraph looks like plagiarism. If you look closely and find revision 3, you'll see I edited a portion into that answer. However later I decided to post an answer of my own, so I took the part that I edited and saved it into my own answer.

In case something similar happens, I would like to know if

1. I commented on another's answer, they edited into their answer, and later I merge that comment into my answer.

2. I edited another's answer to add information, later I use that very info in my answer.

3. Even worse, I remove what I edited into another's answer and put into my own.

Are they considered good, or courteous?

Answer 1

Plagiarism is using the original work of another person without properly indicating that it is not your original work, and who the actual authors of the content are. If someone else cites your work, that doesn't prohibit you from providing your own original content, nor are you required to cite yourself when posting your own work (on SE at least).

Also note that you shouldn't be editing significant original works into another person's (non-Community Wiki) answer to begin with. Now in the case of your particular edit, you're just adding a link to an FAQ, which I wouldn't consider, "A significant original work", so none of this is really relevant to your specific example to begin with, as your edit isn't content that would be appropriate to cite to begin with. Even if you didn't comment/edit on that answer at all, and that answer's author added in that link to the FAQ, you wouldn't need to cite that author for including the same link to the FAQ in your own answer.