I think you might be starting with julia --project and then trying to dev inside that project.
dev is something you do from a different environment. Once you are in that folder you don’t need to dev.
For instance, I just did ] generate test, then closed out of julia and cdd into test and did julia --project. Then I did dev "." and got a similar error
(test) pkg> dev "."
[ Info: Resolving package identifier `.` as a directory at `~/Desktop/test`.
Path `.` exists and looks like the correct package. Using existing path.
ERROR: package `test [c9d88495]` has the same name or UUID as the active project