Children's novel: Children create slightly imperfect duplicates, starting with mug

Question

Children's novel in English, read late 80s/early 90s, I believe recent at the time and set in the present.

A group of children find a way to make duplicate of objects (this may have involved immersing the object in some sort of mysterious clay or mud where the new one appears). The first object they duplicate is a ceramic mug, the copy is almost perfect except that it says "Mad in USA" (may not have been USA) rather than "Made in USA" - a man whom they've asked to examine the duplicate (without saying how they got it) spots this. Later they try duplicating money.

Things escalate and get darker and in one late scene the POV character (I think a girl) discovers another girl has duplicated herself.

Answer 1

Not a perfect match but I did a quick google search for "children's sf novel + duplication" returned a hit for a 1988 children's sf novel called The Duplicate by William Sleator. There is no mention of a mug or clay but it broadly fits what you describe. Seems to have a male protagonist rather than a girl but the synopsis does make a mentions of a female character in jeopardy.

In the story the main character finds a way to reproduce any living object and makes a duplicate of himself. You can read the full wiki entry here.

https://en.wikipedia.org/wiki/The_Duplicate