Copy or duplicated text box not longer editable
Any text box which is copied or duplicated becomes uneditable.
This recently happened to my coworker, and when I tried her doc on my machine the bug was still present. She had to reboot her computer to "fix" it.
That being said, and knowing that some folks like to work with frame edges hidden: if the text box is part of an InCopy assignment or is a linked story, and has been checked out, its duplicate will be "unavailable" for editing. It would need to be unlinked for access (or, the original text box could be unlinked first).