Translation:Good evening, Samia! Good evening to you, Rania!
What they're trying to do is replicate the call-and-reply feature in English. It's a bit formal but rather replying to "good evening" by simply repeating the greeting, one can respond, "good evening to you!" It's one way to remind us that you'd never start the exchange by saying /an-nuur/ and (formally at least) wouldn't reply /al-khair/
I don't quite understand though. In English, I usually don't interpret these as having any different meaning. The only exception is I would use "good night" more as something to say before someone went to bed. When greeting someone, I would use these interchangeably, and I use "evening" and "night" interchangeably more generally. In Arabic what is the distinction between "evening" and "night" in meaning? When would you use each of these greetings?
AlexHrezew - tizerzert - ouphrontis - KatieC,
1) Answering the AlexHruzew's legitimate question, "samiatan" is grammatically not correct either in MSA or Classical Arabic. After "yaa" harf nida, the "samia" word (with the last ending sound) should be spelled as "samiatu". It's the Formal/Standard method.
(Noted that tanwin is not always "a marker of indefiniteness" - many names can take tanwin, for example: محمّدٌ muHammadun.)
Also, pronouncing only "samia" is not a "should be" thing but a less formal way. That's how Duo contributors teach us. They thought that implying the last ending sound would be more complicated and wouldn't gladen our hearts, ... Perhaps! :)
2) All Mr(s) Anonymous/Random Downvoters Without Any Feedback for legitimate comments are the best contributors of the Duolingo's Forums! They have successfully demotivated my learning spirit, indeed - should I say Goodbye for Duolingo's now?