CUPS prints n² as many copies as I want

FWIW, I had the very same issue with a Brother QL-1050 label printer, under Debian Sid. It was not an application bug as suggested in comments, but a CUPS/driver issue. You can confirm this by running lp or lpr and see if it is affected as well :

lp -d YOURPRINTER -n 2 /some/file.pdf
lpr -P YOURPRINTER -# 2 /some/file.pdf

I managed to solve the problem by editing /usr/lib/cups/filter/brother_lpdwrapper_ql1050, and modifying the line

CUPSOPTION=`echo "$5 Copies=$4" | sed -e …

into

CUPSOPTION=`echo "$5" | sed -e …

(Copies=1 also works).

I guess the number of copies was feeded twice somehow.

There must be a similar file for your printer, and though I guess the name and definition of CUPSOPTION may vary, those options are probably defined there.