"When enough coincidences happened one after the other, they stopped being coincidental and started to become unavoidable."
Maybe "Fukagawa" is a common family name. Maybe "Miyu" is a common given name. Maybe every birthday is just as common as every other. But for all of those to match up and for her to look almost exactly like the original Miyu? That's a bit much to just brush off as unimportant.