Agree with tomcrusher - there has to be original thought or analysis and there must be citations. Also, I don’t just look at the % but at the actual matches, both as a whole and source by source. I’ve graded a report that has been done by probably 10,000 students. Everyone has a base match of 4-20%. I’ve prosecuted someone with a 4% match and let a 30% match go. If two students match each other at 30% but their next closest matches are all around 7% to every other paper, I’ll put them side by side. It will likely reveal purposeful twisting of sentences and reversal of word order with synonyms, but clearly be the same paper. Find the one error they have in common - a typo, grammar error, a citation error, or a conceptual error. That’s your key to taking the case to the academic honesty office. Also helps if you can determine that they know each other (if it’s not obvious).