You would say pieces of evidence. It is non-countable. It is a catagory such as fruit. I see the evidence against him. I ate the fruit on the table. It could be one piece or many pieces of either one. And it can be different kinds (a photograph, a fingerprint, apple, banana).
Duty is different. it is not a catagory. And it is just something you have to learn one-by-one. It is countable, similar to the word "tasks". You can have many tasks or duties. They are all requirements that need to be completed.
Hope this helps,