I create a three-page document using Abiword and save it as pdf.

I then open it with Evince and print it on a duplex printer, everything
is well as long as I print one copy.

If in Evince's print dialog I choose 4 copies, I get two sheets with the
first page on front and back, two sheets with the second page on front
and back, two sheets with the third page on front and back.  This is
generally useless: I expect four copies of the document, each copy on
two sheets of paper with the back of the second sheet left blank, for a
total of eight sheets.  The same behaviour is visible if I choose to
print on Cups' PDF printer.

If I tick "collate", I get what I expect: the document is printed three
times in succession, but without any intervening blank pages, so the
first page of the second copy is on the back of the third page of the
first copy.