Another sentence said "the newest dates", I wrote as "die neusten Daten" which was accepted.
Now its telling me "neusten" is incorrect and theres suddenly another E - neuEsten!
I don't understand, both phrases are plural with an article, why does the adjective change? Was there a mistake by duolingo, and if so which word is correct?
If one had two stacks of newspapers--one printed this morning, and the other printed in 1942--and asked someone to point to "the newest newspapers", they should point to the ones from this morning. If you asked them which ones were "the latest newspapers", they should point to the same stack. If you asked them, "Bitte, welche sind die neuesten Zeitungen?" they should point to the stack printed this morning.
I do not know if "die letzten Zeitungen" has the same meaning.
And for what it's worth, myra advised cephalium four years ago, in the comment just below yours, bleusclues, that "We're accepting both 'latest' and 'newest' here." It always a good idea to read through the already-posted comments.
The primary reason I would avoid "newest newspapers" is stylistic, the seeming redundancy (although the "new" in the two different words are not the same concept). I might, however, say "The newest papers," if contextual clues made it clear that I was talking about newspapers and not some other printed documents. I think die Eule accepts the shortened form of newspapers here.