The IDE testcases and the validation testcases look similar, but they serve very different purposes. For a beginner it's not easy to see the difference. Sadly Codingame only tells you the validation testcases are there "to prevent hard coded solutions". That's certainly right, but it doesn't explain anything.
Lets start at the beginning. The problem description tells you what your program has to do. To solve a problem, you have to write a program that produces the right output for every possible input.
The IDE testcases are there to help you understand the problem description and to facilitate debugging. They are examples and tell you what output your program has to produce for a specific input. That's why you can see the input, the output and where your program produces errors.
The validation testcases serve a different purpose. To see if you have solved the problem Codingame would have to check that your program produces the right output for every possible input, but that's obviously not practical. Instead, Codingame only checks for a limited number of inputs (the validation testcases) that your program produces the right output. That's the reason they can't give you any information about the validation testcases.
So what does it mean if your program passes all IDE testcases, but fails some validation testcases? It means you have to go back to the problem description and think about your program. Because for some input it doesn't produce the right output. A good way to go about it is to make some more testcases yourself.
For example take your "duplicate mime" testcase and look at the IDE testcase. It's probably a list of mime types with two identical entries somewhere (e.g. a,a). Here are some questions you can ask yourself and use to construct additional testcases. Does your program also work if: there are three or more identical entries (a,a,a)? there are multiple identical entries (a,a,b,b)? the identical entries are far apart, or at the front/end of the list? all entries in the list are identical? the duplicates are not in order (a,b,c,a,b,c)?
But these questions obviously depend on your program and you have to find the right questions yourself.