Imagine a situation where several people are sent to an interview and only one has to pass. His knowledge is enough for the project. However, the customer asked for the several candidates so he can choose the best one. And the other people from your company are already engaged in other projects and should not get the job.

The question is: How to show that the other candidates are worse and at the same time not make them look stupid?

UPD 

I've got the point but I need to clarify the primary idea. I wanted to know how another candidate [that should not pass the interview] has to behave at it? 

I've left the modified question [1st one] because several answers correspond to it.

If you haven't done so yet, tell the customer about the situation up front. If he still insists on interviewing the other developers, let him do it, in a fair manner (i.e. all developers answer honestly and to their best ability), and let him compile her order of preference.

Then let him know the cost (in time and cash) of transferring each developer to this project (including a replacement in the other project), e.g.:


Bob (engaged in the Foo project - estimated replacement cost $50K and 4 weeks)
Jane (engaged in the Bar project - estimated replacement cost $30K and 6 weeks)
Jack (can start immediately)
Nat (engaged in the Groo project - estimated replacement cost $80K and 10 weeks)
Mary (can start immediately)


As long as he is willing to pay the associated price, he can choose whichever developer she prefers.
