Good question! First, I think you mean punished not published?! I believe the will in the sentence emphasises the fact that the captives may know which one is Spartacus but may choose not to tell the general. So, will = if you choose to tell. Could also ‘would’ if the promises was ‘promised’. Hope this helps. :)

Rohan Iyer is right. The meaning of 'will' in the if clause is 'if you choose to, agree to, are willing to'. Generally, if clauses do not contain will. But they can with this meaning, e.g. 'I'll make lunch if you'll make dinner "

"Will" is a verb (remember what verbs do) that signifies future tense. Since he is saying, "...if they will...", they are showing a future event to possibly take place. However, since the first "will" is already indicating a future tense, it's ok in most situations to drop the second "will" in this scenario if the intent is for them to identify Spartacus now. If he is saying they might see Spartacus next week or sometime in the future and they need to identify him at that time, it would be proper to keep the second "will" in the sentence. Also, remember in the Arts, such as movies, poetry, theatrics, the rules of English are sometimes pushed to their limits and sometimes dont follow normal rules. This example seems pretty straightforward but I just wanted to mention that for future reference. I hope this helps! Feel free to contact me if you need further explanations. Take care.

I’m a native speaker and I don’t see anything wrong with any version. In fact the one with the “will” in it sounded weirder.

The second "will" in this case is used as a means to emphasize that if the captives cooperate their incentive is not to be punished. It is like someone says if you "choose" to get me a cup of coffee then you may leave work early. Some people will raise their hands to make quotation marks with their hands particularly when they are say a certain word they want to emphasize.