In this case, I wanted to paraphrase the answer a little bit, but what should I do when the cross site duplicate is exactly as I think it should be? Copy paste it and then link to the authentic answer at the last sentence? If not, then what?
Yes, that's fine. Giving the overlapping scopes of some of the sites in the network, there are bound to be some duplicate or near-duplicate questions, and thus answers. Paraphrasing and adjusting the answer so that it's more appropriate for the site is nice, but sometimes it's just not necessary.
According to CC BY-SA 3.0, the license applicable to all Stack Exchange content, you have to show and hyperlink the author names, but in practice that's often omitted.
It's nice if you use blockquotes (
>) to visually indicate that it's a quoted answer instead of your own words. It could get messy with the nested quotes, but I think it'd work:
As noted at the foot of http://www.daemon-systems.org/man/lseek.2.html:
A seek() function appeared in Version 2 AT&T UNIX, later renamed into lseek() for ``long seek'' due to a larger offset argument type.