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You may want to check out the Steem Links community where that type of post is highly valued.

I personally follow etiquette that goes something like this:

a quoted paragraph or two, and an image or two can be posted
a brief summary of the entirety of the article you're referencing can accompany the quoted text, or stand on its own
attribution should always be given to the external post's author(s)
a link to the source is necessary
with permission from the author, an entire post could be reproduced, but it should be obvious that the post originally appeared somewhere else, and that permission to reproduce the material has been granted by the author(s)

I think that in most cases these rules generally apply, but various communities may adhere to a different set of specific guidelines.

There have been a number of posts on Steemit regarding etiquette and plagiarism specifically. You may want to try a search to read the approach that users expect of each other. One post of note is Achievement 3 : Content Etiquette in the Newcomers' Community. Perhaps you'll find some helpful guidance there.

Cheers. Steem on!