When two cells in the same unit hold exactly the same two candidates — say 2 and 7, and nothing else — those two digits are spoken for. One cell will take the 2 and the other the 7; you may not know which way round yet, but no other cell in that unit can have either. Erasing 2 and 7 from the rest of the unit often cracks it open.
r5c2 and r5c6 lock digits 2 and 7 for row 5 — the red candidates elsewhere in the row are removed.
In row 5, cells r5c2 and r5c6 both show exactly 2-7. Those two cells will take the 2 and the 7 between them, in some order. The 7 pencilled in r5c4 and the 2 in r5c8 are therefore impossible — erase them, and watch r5c4 collapse to a single.
The pair must live in one shared unit. Cells r5c2 and r6c6 with identical candidates share nothing and prove nothing — geometry first, candidates second.
These grids come from rated games on this site — our solver confirmed that naked pair was part of the shortest solving path for each one.
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