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The Agree button is there regardless of whether the ID is the same as the one you already provided, see https://forum.inaturalist.org/t/why-am-i-asked-if-i-agree-with-an-id-thats-the-same-as-my-original-id/17071 for previous discussion.
If you click the Agree button, it will withdraw your first ID and add another one for the same species at the bottom of the list of IDs. Since your ID would no longer be the first ID of that species, it would change from Improving to Supporting.
The fact that one has parentheses and the other doesn’t could be considered a bug, but may not get fixed until the notifications revamp.