Is this [duolingo example] a valid sentence?
Mit ihnen stimmt doch was nicht
There's something wrong with you.
Is this a valid, correct sentence that Germans would use? A Google search yields less than 500 results world wide.
Secondly, if it is a valid sentence that Germans would use, could somebody please explain it as it makes no sense to me?
That version is a bit informal. The 'normal' one would be: "Mit ihnen stimmt (doch) etwas nicht" = "Something is wrong with them". You will find a few thousand google results making combinations with doch, etwas, was, dir, mir, ihm...