How to penalize students who copy each other but don't realize they have plagiarized?
Learn about your school's policy. Learn about whether your school has an honor code, defines plagiarism in its catalog, or requires mandatory training such as an online orientation. Many schools have specific rules about what kind of punishments are allowed, and these rules may be based on their own interpretations of local laws or their past experiences with lawsuits. For example, my school's policy specifically tells us that we are not allowed to give a student a failing grade in the course for dishonesty, and it also spells out different levels of consequences for a first and second offense.

Look at your own syllabus to see what you said about plagiarism; usually administrators will support you on anything that's laid out clearly in your syllabus.

Don't assume that administrators will support you. Their priorities may be very, very different from yours.

Once you have done all your homework, talk to your dean and propose how you want to handle the issue. Make sure your dean is on board with what you plan to do. Consider "soft" penalties such as forcing the students to meet with the dean.