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.
The company I work for has about 15000 employees. Every now and then they have a doppleganger section on the internal homepage. People send in the names of workmates they think look like others in the company and their work ID photos get posted side by side on the site. Amazing how many 'matches' there are.
So far nobody looks like Dilbert.