Verify that the token receiver ('to') do not have already a token for the event ...

[todo]

https://github.com/FightPandemics/FightPandemics-Web3/blob/79da78e91bab545fea9eee31966fb97bacdec1aa/POAP NFT#L207-L212

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This issue was generated by todo based on a TODO comment in 79da78e. It's been assigned to @Islandersfan2016 because they committed the code.