I\'ve recently had the case that a user un-installed my Shopify app and instantly re-installed it. This caused a problem because I store all my users in a DB table.
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I've had this problem with my apps as well lately. Webhooks only started getting delayed in the last 2 months, and I'd be surprised if most apps out there weren't suffering from this regression bug now.
The way I deal with it is - when the user is redirected to the app and the old db object/token is still present in the database, try calling a dummy API call to the Shopify API (something like get shop details) with the token you have. If you get a 403 Unauthorized response, invalidate the user session and refresh the stored token.
Another problem is that after a minute or two when the original uninstall webhook does fire, do the same procedure - check for a 403 response. If you DON'T get a 403, then you know that the webhook is old and shouldn't be acted upon, because if you get a 200 OK it means that your token is good and that the app is still installed.
It's a bit convoluted and it added a fair bit of code to my apps, but it's the only thing I could think of on a short notice - because merchants do uninstall/re-install quickly fairly often.