On Apple Computer in both Library and on the film strip in the Develope module LR automatically selects two files instead of one.

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1. Are you using a US English keyboard or another language’s keyboard (e.g. French AZERTY)?  

2. Are you using the keys on the numeric keypad or the main keyboard?

3. Please do the LR menu command Help > System Info, click the Copy button in the upper right corner, and paste into your reply here, so we can see exactly which versions of hardware and software LR thinks you're running and important LR options that are set.

Moderators, ​@Rikk Flohr_Photography, please merge with the duplicate post:

See my reply to your other post:

First, restart your computer. If the problem persists, try resetting LR's preferences:
https://www.lightroomqueen.com/articles-page/how-do-i-reset-lightrooms-preferences/
 
LR sometimes soils its preferences file, and resetting it can fix all sorts of wonky behavior. That article explains how to restore the old preferences if resetting doesn't help.

Check if you are in ‘compare’ mode.

Post this in the Lightroom Classic forum (you may need to change the title slightly to avoid a duplicate post error):

https://community.adobe.com/lightroom-classic-672

I am using an Apple studio computer.  In both the Library Module and in the film strip, when selecting a file, the next file is also selected.  When trying to scroll through the other pictures in the folder, it only scrolls between the selected two files.  This problem started after updating to the latest version of Lightroom Classic.