"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.
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further explanations. Take care.