Wait, the resemblance thing is also used in other languages: “spitting image” in English, for example, and “copia sputata” in Italian. I’m actually wondering for the first time where it comes from, so maybe there’s a reason for the Portuguese saying to be related to spit
From a quick search that didn’t provide anything really insightful, it seems that at least in Italian the term has been used since the XIV century, so it’s not photocopy related