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How do I do that?

I meant you sort of double confirm already. The year and the month combination is no doubt any more. There is doubt if one born in month of January or first few days of February.
It's kind of joke when I hear my fellow country man and woman said double confirm. I always thought confirm is confirm, what is double confirm? But it's the kind of obscure English that grew with us.
Maybe from 'double check' to 'confirm' and someone must have added 'double confirm'. :-)
If you know have anyone who wants to confirm, check the day they are born versus the Hsia Calendar.

The person must be born on or after the first day of spring according to this calendar. That is mostly 4th February but sometimes 3rd and sometimes 5th. For this year, it's 3rd. That means anyone born on 3rd Feb is born in the year of the rooster. One born on 1st January to 2nd Feb will be born in the year of the monkey. https://en.wikibooks.org/wiki/Ba_Zi/Hsia_Calendar