What does “have seconds” mean?
Addie remembers the way the nurses would crowd around to look at them as if they were animals in a zoo. The chocolates would come out and they’d be encouraged to have seconds.

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meal? but it seems that the nurses only give those two little girls chocolates, in order to entice them to talk?

so, shall I say that here , in this specific passage,"seconds" mean "another chocolate?" 
thank you!

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When you have seconds, you have a second helping of a meal. You should not think of it as 'have more than one. I can have more than one sweet, but that does not mean I am having seconds, I am just having another sweet. Seconds is very specific, it means another serving of a meal.

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Have more than one.