This is an interesting case. If people are free to place the coins as they wish, I wouldn't be surprised if P(heads)>P(tails) due to biases about which way is "right side up". When a face appears on a coin many people seem to think of this as the front of the coin, and in numismatic circles, the obverse is typically "heads" if a head appears on only one side (although no surprise that there are contentious debates about obverse/reverse in specific circumstances).
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