If I click "Thank" on a post in Flow, the UI word changes to "Thanked". If I close my browser, and then reopen the board or topic (e.g. the next day), the UI word is once again "Thank" (which is confusing to me), and clicking it will send a second notification (confusing/annoying for the recipient).
The same applies to Thank links in wikitext history pages, e.g. https://en.wikipedia.org/w/index.php?title=Wikipedia_talk:Notifications&action=history
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This means I have to rely on my memory, as to whether I've ever thanked someone for their post/edit.
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There are a few possible solutions:
1) Clicking "Thank" on a [post/edit] I've already thanked, would trigger a popup message saying "You've already sent thanks for this [post/edit]."
2) Change the "Thank" to "Thanked" automatically, on each page-load. (It would have to stop relying on client-side localstorage, which is full for some powerusers.)
3) ...?