LR selects 2 photo's

[Gert-Jan van der Vlis]

Since today LR selects sometimes 2 photo's (not in compare mode).

1 photo with a white and  1 with a black mark in the upper right corner.

Due to this problem it's no longer possible to choose new photo's with the cursor keys. It's only switching between the 2 selected photo's.

[GoldingD · Accepted solution]

Those itty bitty diamonds in the upper right of the thumbnails mean that you are in compare mode. (Keyboard shortcut C)

[Gert-Jan van der Vlis]

My 2nd screen (switched off) was in compare mode.
Thanks for your tip!

[AxelMatt]

If pressing the shortcut C to get out of the Compare mode doesn't help try to reset the preferences of Lightroom Classic:  How to reset Lightroom Classic preferences (adobe.com) 

It's recommended to backup your preferences before you reset the preferences to the default settings: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html