Clarifying since some didn't notice I said "even if you are allowed to have multiples".
This means whether it is "unique" or not.
Couple examples to clarify since people need specifics:
Cottontail tokens, I "won" 4x upper body, this is NOT unique as I already had 3 tokens in inventory this was obvious. I don't recall the tokens on this one but I chose to cash it in for the shown amount which was an additional 4x the oddments originally awarded. No additional tokens were received.
Since that one and this thread I started paying more attention so the next is more specific.
Last night I "won" 4x bad weather umbrellas, this has a 500 oddment award with it and showed 1500 oddments for exchange (I am guessing this is 500 for each additional item). I chose to exchange and received only, I did check my oddment balance before each key used to be sure.
A few keys later I "won" 3 bad weather umbrellas and chose to accept the item and the 500 oddments this time. It showed 1k oddments if I had exchanged but I had chosen to keep the item and didn't expect more than 1 from past experience. I received one bad weather umbrella and 850 oddments not the shown 500.
So it looks like you get a partial compensation for the extras if you accept the item and get no extras if you choose the exchange.
This doesn't happen often as not a lot of TH events award multiples and there is no clear information on what to expect from this so it's pretty much take what you get or don't play.
To me this appears to be a bug and I can't imagine how Jagex would be unaware of it by now, it is likely not a priority with everything else on their plate. I'd have to agree with that assessment. Should be fixed but there are much more important issues.
If this post persists and I see anymore happen I will append this reply with specifics.
12-Sep-2019 00:11:32