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Any ideas on the measures being made to combat this? Subjective proof of work using manual labor for double-checking seems to be a race against spam..

Not the most effective, as accounts have to be singled out, but a first step:
https://steemd.com/spam/@pharesim/another-test

Someone willing to write a plagiarism searchbot? :D

Might wanna dig my hands into it, what are you using atm?

Simple user blocking right now.
I got in contact with a commercial plagiarism check api provider and will play around a bit with that one over the next days, comparing it too google custom search api.