A short python script to concatenate several .pdf files into one so that they can be printed duplex. Uses gs and pdfinfo.

printMe.py

import subprocess

fileStack = []

with open('./printMe') as f:
    files = f.read().split('\n')
    for file in files:
        if file == '':
            continue
        fileStack.append(file)
        proc = subprocess.run(args=['pdfinfo', file],
                              stdout=subprocess.PIPE)
        for output in proc.stdout.splitlines():
            if output.startswith(b'Pages'):
                pageCount = int(output.split(b' ')[-1])
                if pageCount % 2 == 1:
                    fileStack.append('empty.pdf')

gsCommand = ['gs', '-dBATCH', '-dNOPAUSE', '-q',
             '-sDEVICE=pdfwrite',
             '-dPDFSETTINGS=/prepress',
             '-sOutputFile=printMe.pdf'] + fileStack

subprocess.run(args=gsCommand)

Run with python printMe.py.
The script expects a file named printMe containing the paths to all .pdf files that shall be concatenated.
For concatenation a blank page may need to be inserted.
The blank page is expected to be available as empty.pdf.
The produced file will be named printMe.pdf.