Question: I just cut an object in half and then mirrored it. 
Before i joined the objects, they looked perfectly similar, but after joining, one half is darker than the other.
I tried some stuff and i did delete doubles, and i checked the material
but it all doesn't help

That's the thing, after joining the 2 halves:
[![thats the thing, after joining the 2 halves](https://i.stack.imgur.com/hDSH7.png)](https://i.stack.imgur.com/hDSH7.png)

Answer: 
I have 2 possible solutions. One, try (in edit mode) pressing {control n} which will recalculate the normals. If the normals are incorrect then it will fix that. Otherwise, it's just a question of shading.
My solution is. Roughness set it to 1.0.
I'm creating character, I really don't know how to do. But when I did it. It goes fine.
Maybe after applying Mirror then that's the time to adjust roughness again if you need.