When creating an assignment in Google Classroom and attaching documents you have 3 choices for how to attach the file. View, Edit or Make Copies. There have been times that I have published an assignment and accidentally made a file View or Edit instead of Make Copies. I have also accidentally forgotten to attach the document that I needed to Make Copies of. Unfortunately, after assigning you no longer have the ability to add a document as "Make a copy for each student."
Note: In Fall 2018 Google Classroom made changes to the interface of Google Classroom. There are some changes with the screenshots below. The reuse post option is now on the Classwork page. You still use the 3 dots to edit.
To edit an assignment in the Google Classroom Stream, click on the 3 dots in the upper right-hand corner.
When editing an assignment, you are able to add additional documents. However, you only have 2 options for the file: View and Edit.
The trick is to reuse the post from the same course. Click the plus icon in the bottom right-hand corner and choose "Reuse post."
Select the SAME CLASS that you are using. You can reuse posts from current classes, other classes, classes you are a co-teacher in, and archived classes.
UNCHECK THE CHECKBOX
This is super important. Make sure you UNCHECK the "Create new copies of all attachments" checkbox. Your files are in Google Drive. You do NOT want multiple copies of the same document in your Google Drive.
Edit the Post
The reused post can be edited. Any documents attached, or new documents attached, have the option to "Make a copy for each student."
Delete the original assignment only after you have reused it.