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The use of sendmany in a PoS coin is an obvious source for preparing stake inputs, however it historically has been prevented to send to duplicate addresses in one transaction with such an error as "Invalid parameter, duplicated address:"
It seems like a reasonable issue in BTC, but are there any good reasons to prevent this?
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This is a limitation on the interface used for send many. The address,amount list is a dictionary internally within Bitcoin Core, and cannot have duplicate keys. As many other projects are forks of Bitcoin Core, the limitation applies to them as well.
You can work around this by constructing a transaction manually or with a library that doesn't not use the RPC interface, nothing at the actual protocol level prevents this.
