I have also seen this, primarily with photo contests. More than once. It is a little frustrating because while I joined those contests, I had no chance of winning with great talented photographers posting original pictures plus plagiarizers posting amazing stolen pictures. I too have been able to catch this, primarily with pictures.

I've also falsely accused someone but I was able to quickly realize my image search had taken me to that users personal website so I was able to change my comment.

If anyone cares to know, you can perform Reverse Image searches. This takes the "Steemit URL" of the picture in question and searched billions of pictures for a copy. In most cases, original content will come up with 0 results. Sometimes the photographer has their own website and are sharing old pictures so you might get a few hits.

Two reverse image websites I use frequently:
Google Reverse Image Search for mobile phones
https://reverse.photos/

TinEye - Lets you paste the image URL. Very handy for catching plagiarism!
https://www.tineye.com/

Good luck and let's keep steemit original!