Split from T120152: Bundle "Thanks" notifications
'Thanked' status is not persistent per user - e.g. a user who 'Thanked' in one browser, will see that 'Thank' can be done again when he'll go to the post in another browser.
The same happens, of course, when cookies are cleared.
After 'Thanked' a user re-sort topics on Flow board, 'Thanked' will be displayed as 'Thank' again.
The page e.g. Special:Thanks/Flow/[revision number] is displayed quite unexpectedly when multiple 'Thanks' are performed or with a double-click on 'Thanks'
An edge case: if a user has two accounts, logging as a different user after 'Thanked' was done from another account, will display the post as 'Thanked'. Reloading page is no use - 'Thanked' stays. It switches to 'Thank' only if the topics are re-sorted or cookies are deleted.
Note: It's only in front-end; the back-end sends only one Thank notification.