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With the Android app, open the sidebar then click on Edit near Your sites to open the menu of pinned sites.
This open this page:
Clicking on the bottom arrow opens a list to add sites to the list of pinned sites:
See how Stack Overflow (and other sites) are present in both images? Why does the app offer to add Stack Overflow (and others) if it's already in the Your Sites list?
The same issue is present on web. This makes me think it may be status-bydesign.
However, sites already in "your sites" are given a darker, shaded background.To be consistent, a similar behavior should be used in the apps.
