Can I show the same coin twice?
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Yes. Put the same coin in two or more boxes and it takes those turns in the rotation, so it is on the glass more often than the others. The page points it out under the boxes rather than stopping you: Bitcoin appears more than once, that's fine, it simply comes round more often.
Two different coins that happen to share a symbol are another matter, with their own warning; see Coins that share a symbol.