I've notice that, on more than one occasion, there is a difference in the computer voice between emphasized syllables and grammatically stressed syllables, which you can determine by the pronunciation of "o" as "o" instead of the unstressed "o" = "a" sound. The audio is pretty fast here, so it's hard to hear "dah-rah-goy" - and the computer is giving the wrong emphasis to the 2nd syllable. Here's a better computer voice pronouncing the word: https://translate.yandex.ru/?lang=ru-en&text=%D0%B4%D0%BE%D1%80%D0%BE%D0%B3%D0%BE%D0%B9
And it's even more clear at https://forvo.com/word/%D0%B4%D0%BE%D1%80%D0%BE%D0%B3%D0%BE%D0%B9/#ru
I usually report these as "problems with the audio", which you can often do even when you get something right.
But - and that's a big BUT - I believe that Duo has little control over how the audio functions in these matters. It's unfortunate, because Duo really does a very poor job teaching people how to speak Russian phrases and sentences.
The only real way to learn a language with any degree of fluency is to speak it. That's the key to internalizing words and grammar.
If the word "дорогая" uses with the person's name then it shows that you appreciate the person. You may use it when you appeal to a person, for example:
"dear Maria" - "дорогая Мария".
Unnamed - is the form of address to each other between spouses or people in couple:
Dear, please buy bread - дорогой (/дорогая), пожалуйста купи хлеб.
If you say so about a girl - the phrase "дорогая девушка" is material in nature. Usually such say about hookers.