I know you've closed the question, but either the interface was tweaked or I just now realized how to tell:
If you thank someone & the text on the page that opens starts with "Thank you for thanking [whoever it was]", then that's the first time you thanked the person for that action. If it just says your thank you has been recorded, it means "already" but it doesn't say that anymore & the visual layout is not noticeably different, like it used to be (I use the already thanked feature to tell when I've last been active in interacting with someone through "thank yous" & was annoyed when it appeared that I was getting to the exact same page whether it was the first time or the 20th that I'd clicked that link [my eyesight is not good enough to easily distinguish the clicked on/not clicked on colors]).
Anyway... now I just need to glance for that bold "you" at the start of the text & I know it's the first time if it's there (and a repeat if it's not).
Hope that helps.