How to switch off automatic stacking of photos?
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When are photos getting automatically stacked? If you're referring to raws getting automatically associaterd with their corresponding JPEGs when you import, you can disable that by checking the option Preferences > Treat JPEG Files Next To Raw Files As Separate Photos.
If that's not what you're referring to, please provide more details about what you're observing -- which types of photos are getting stacked, when does the stacking occur, which version of LR (do Help > System Info).