Hey there,
I am doing a Render of a Plastic Bottle:
So I hope you have some ideas to solve this problem.
Thanks in advance! 
Try making backfacing faces invisible. If the mesh is completely closed, this should work
So what do you mean with “making backfacing faces invisible”? You mean hide the inner mesh?
I’ll show you what I’ve got:
I Just made a quick render with just one mesh:
With “just one mesh” you mean a mesh without thickness? That will most definitely look weird because Cycles now only “sees” one outer surface with normals pointing outward and assumes that bottle is a solid glass object (= not hollow).
However, I somehow don’t see that double reflection issue in my experiments with glass objects: So, what is your material setup for the glass material? Could this be a scale issue (= Your bottle is not 10 meters tall, is it?)?
Hey there, thanks for your reply!
No it isnt 10 meters tall 
Scale and Rotation are aplied too!
Instead of copying the whole mesh, I extrudet it and ended up with this:
Its just the Glas shader on the bottle.
With an IOR of 1.200 this time.
Its way too reflective as I think.
Yeah, that’s interesting, because I always have the feeling the pure glass shader is not reflective enough…
That’s why I like to use a mix shader of glossy and refraction (instead of glass) because that allows me to better fine tune and balance refraction and reflections (see example and shader overview below).
In your case the bottle would be even less reflective - looks rather like a plastic (PET) bottle than a glass one, no?
I suspected this might be the case but I just wanted to confirm it, and I hadn’t the time until now - the vertices on the inside of your bottle have their normals flipped, causing the second reflection. On the left, the ‘normal’ normals. On the right, the internal normals are flipped. Looks nicer in a way, doesn’t it? To correct this select all the vertices and recalculate. Then you’ll have just the regular dull reality of polyethylene.
Hey Guys! First at all thank you for your support!
I started from scratch and instead of copying the mesh I used the Solidify-Modifier.
And I made two testrenders with the normal Glas Shader and the Shadersetup from Ikari.
