As an aside -- I have always found this type of sentence formulation, common with software engineers, so mystifying. It would be just as easy to understand without the sentence variables -- it doesn't add clarity but it does add length.

The alternative:

> Once in our CS department (not CMU), we had given PhD acceptance to a wrong student, since he carried the same name+surname with the candidate we intended to admit.

> Instead of rescinding the acceptance, we let both join. It turned out that the candidate we admitted by mistake performed much better than the other. :-)