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Yes, the new chain will wear prematurely.
Fine is relative. Supposing it shifts properly: If it’s a cheap chain and you want a working bike now, it’s fine; If you’d rather not pay for another chain soon and/or you’d prefer not to have a bike that suddenly stops shifting properly, then it’s not fine.
Advice for that level of wear is to get them both done together. A bit more money now for a lot less hassle and the same or more money prematurely down the line.
While you’re at it, have you checked the chainring(s)? If they’re gone, they’ll wear the rest prematurely too. Chainrings can be expensive enough that it’s worth buying an entirely new groupset, again depending on what you have.
Will riding a new chain on a cassette that previously had an old chain measuring >.75 on a checker cause premature wear? Or as long as it shifts OK, is it fine to use?