I've tried modified EVERY element for the assignment but cannot figure out why the option to duplicate is sometimes unavailable. I have only run into this issue recently (within last two weeks)!!
[examples are from the same course]
Without know exactly the type of assignment or resource you are referencing in your post, it is hard to know for sure what might be going on.
My understanding is that Canvas will allow you to duplicate assignments, discussions, modules, new quizzes, and pages, but not announcements, files, old quizzes as well as assignments configured as external tool submission type.
See the note on this guide.
Here is what the assignments look like on my modules page:
Here are the options when I click the first one:
Here are the options when I click the second :
Here are the details for the first one:
Here are the details for the second: