Git stores the contents of a directory in a tree object. In general, tree objects are supposed to be in sorted filename order by byte value.
This message means that those particular tree objects contain a duplicate entry, which isn't supposed to be the case. Whatever tool you're using to create these objects has created corrupt ones, and Git is now ...
It sounds like you are new to Linux.
I may catch some heat for answering this, but the error message is telling you that you are not running the command with root permissions (similar to "Run as Administrator" in Windows).
In order for you to run commands with root permissions you must add the command sudo to the beginning of your command string. So what ...
Another way is possible if you're using tree 1.8.0 since it supports the --fromfile flag:
--fromfile Reads a directory listing from a file rather than the file-system. Paths provided on the command
line are files to read from rather than directories to search. The dot (.) directory indicates that tree
should read paths from standard input.