Hi Emufarmers!
Thanks for your attention! :) Yes, the tool might detect the sentences that already referenced, because Citation Detective actually feeds *every single sentence* in an article to Citation Need model and extract sentences with high scores.
Highlighting a sourced sentence doesn't mean the source used is unreliable as Citation Detective has no idea of whether a sentence has a reference and the content of the source. I would say it's just like double confirm that the sentence needs a citation, and yes there is a citation already.
You might have questions why the tool didn't exclude those sentences already have a reference. The reason is a reference doesn't necessarily apply just to the sentence right before it. They could apply to more than one sentence or a whole paragraph, and there's no way to determine that from the Wikitext. So that's why the tool was designed (at least for the initial version) to feed every sentence to the model and detect citation need score.
There's the same case with { citation needed } tags, so you would also find sentences with {cn} tag highlighted in the prototype. That means both human and machine think the statement needs a citation to a reliable source.
I hope this clarifies things for you. :)
Aiko