I encountered plagiarism almost immediately when I taught English. Each year, a few students attempted to copy works, writings and even drawings of published authors and artists. The school had a subscription to TurnItIn to identify obvious copying of others' works in academic essays.
There's no such service for fiction writers.
In the last few months, the romance genre has exploded with a plagiarism scheme that involved an "author" copying passages from multiple books and massaging them only slightly. A fan noticed similarities, and emailed an author with comparisons of the text.
Through my interactions on different FB groups, I've learned there are entire platforms for exchanging and critiquing work that are rife with "authors" outright stealing works and selling them as their own.
As a writer, it's certainly something to be aware of. But what can fiction writers do? On various levels, we're reassured that no one will try to steal our works. We're told too much effort would be required and that our copyrights are protected the instant our pencils hit the paper, or we have an electronic time stamp on the files.
Which is true, but...this blog post is the best I've seen outlining the steps to protect yourself and your work. Because you've worked too hard to let someone else take credit for your story!
And as with catching my wayward students, the best defense is being well-read and checking in on the various platforms that publish stories. That way, you or a fellow reader can identify when someone steals your hard work (as in Nora Roberts' case).
The overall solution: We all need to read more.