Cycles Node Editor: Double Diffuse problem Basically there is 1 area and it is all 1 material.
It consists of 3 parts/UV maps: Mountain, Soil, and Beach. The UV maps "overlap" each other which means that the faces of transition between the parts are in both/all three of the UV maps.

The layers of the textures have an order: First there is the mountain, then there is the soil which covers a bit of the mountain, then there is the beach which covers some of the mountain and the soil. The soil and beach textures have slightly transparent parts at the transition.

I use the general Color/Normal/Occlusion/Specularity setup for each of the textures. In the end the different parts shall all have their own diffuse and glossy shaders, the mountain f.e. gets a slightly red gloss to it.



Now there is the problem that the faces of the transition seem to be "diffused twice" you can see that they are way brighter:

(note that it really is just the faces of the transition not the whole mountain even if it might look a little like this in the screenshot.)

Here is an image of the node setup, sadly it is too small to read it:


Last but not least the .blend-file(too large for blend exchange):

Choose one:
http://www.file-upload.net/download-10862405/area.blend.html
http://www.megafileupload.com/5157/area.blend



To sum it up: Is there a way to diffuse it all with the same brightness whilst remaining separate color and normal maps plus separate specularity maps and glossy shaders. Aka "how to set it up correctly".



EDIT: Here are the textures packed in a .zip:

http://www.file-upload.net/download-10862545/textures.zip.html

http://www.megafileupload.com/5176/textures.zip



EDIT: The answer of PGmath might work but in my case it doesn't because the textures are "hand painted" and they already have transparent parts at the borders and stuff. Isn't there another way to achieve this? I cannot use the mask method, I'd have to do the whole thing again.
I think you are going about what you are trying to accomplish the wrong way.  The way multiple different materials are usually combined is by using a mask.  A mask is basically a black and white map where black areas correspond to one material and white areas correspond to another material.

To set this up in the node editor just use a mix shader and plug the mask into the fac input.  This will make lighter areas of the mask give the output shader more of the second input and darker areas give more of the first input.


Note: make sure you set the image texture node for the mask to non-color data.

Here is a rendered visualization of just the mask plugged into a diffuse shader across a piece of randomly generated terrain:


And here is a render with the same mask only in the above node setup:


So for your case you will have to now go and create a mask which defines where each material should be.  To combine more than 2 materials just create multiple masks and use multiple mix shaders in succession.