Saying shehechiyanu by an early Kiddush on the second night of Yom Tov
This year the 2nd night of Shavuos is on Shabbos. This means that al pi din, one can make kiddush early unlike a normal 2nd night. Although it’s not the Chabad minhag, I may have to do it due to circumstances.
My question is, would it make sense not to say shehechiyanu at kiddush, but wait until later after dark? The reason would be because in theory, if one forgot to say it on the first night, he can say it anytime the next day until sunset. This shows that it’s still the first day. It wouldn’t make sense to say it twice in a row, so even assuming that I did say it the first night, can I say twice within 24 hours? This is my own idea; I didn’t see it anywhere, but I would like to know if it has validity?
The same theory could be used regarding Shechiyanu by candle lighting. in practice, however, we do say Shechiyanu. This is because when we light candles or make kiddush we are accepting the 2nd day upon us.
Also, based on that theory, no one should ever say shehechiyanu on the second night, because halachically Shavuos has an extension for seven days, so it’s like saying twice within that time frame.
In any case, the theory is not correct and shehechiyanu is always said.