After reading bchadwick's answer, I'm adding to mine. Two projects can hjave the same discount rate and IRR in only two cases:
1) the NPV profiles are different, but they have the same IRR -- and you calculate the NPVs of both projects at the common IRR (in this case, NPV will be zero). However, for these two projects, the NPV will differ for all other discoutn rates
2) The two projects have identical cash flows in all years. In this case, the two projects are (by definition) identical.
In any other case, either the two NPV profiles never cross (so that the projects never have identical NPVs, or they do cross. But in the lst case, they will have identical NPVs only at this crossover rates. At all other rates, the NPVS would differ (including the rate that forces one or the other to a zero NPV). So, in this case, since they have different NPVs, the rate that results in a zero NPV for project (A) will not result in a zero NPV for project (B). Therefore, it's not (B)'s IRR.