I currently have like 10 tests that test whenever my Tetris piece doesn't move left if there is a piece in the path, or a wall. Now, I will have to test the same behaviour for the right movement.
Is it too bad if I just copy the 10 tests I already have for the left movement and make only the needed changes and do the same for the code itself too? Or should I go again and make each test from the beginning, even so if the logic is basically the same?
I sometimes find myself making very complex unit tests only for avoiding test code duplication. I think it's not good to do so. Any single unit test should be as simple as possible. And if you need a duplication for achieving it - let it be.
On the other hand, if your unit test has +100500 lines of code, then it should obviously be refactored, and this will be a simplification.
And, of course, try avoiding meaningless unit test duplication, like testing 1+1=2, 2+2=4, 3+3=6. Write data-driven tests if you really need to test the same method on different data.
