What about the fact one holds his coffee with the left hand and the other holds his coffee with the right. Combine that with the shirt pocket thing someone else mentioned and I'd say this is his mirror image as opposed to replica.
So the real question here is which one is the real dilbert? In reality, it could be either one. most of us assume it's the one on the left. :P
The only way to tell is to realize that Dilbert's shirt pocket is always on HIS right. (all shirt pockets are, unless it's a girls shirt or it has two pockets) Dopplebert's shirt pocket is on his LEFT meaning that he is either female, or a mirror image lookalike right down to his clothes.