[regularfry]

I'd argue that even a test which is a copy and paste of the implementation method - literally the same code, twice - has value, because you can then refactor the implementation as much as you like in future and know that you're covered. Nothing says that because they are the same now, they will be forever.

[tieTYT]

As someone who has personal experience making this mistake, I disagree with you. You lose the most important purpose of the test: The confidence that your code works. If you write the code twice all you know is that your code hasn't changed.

And in my personal experience, you CAN'T refactor because the test is so tightly aligned with the production code. EG: moving a variable declaration up or down one will break a test even though it didn't change anything.