I don't know which diagnostic summary table you used to pull out the diagnosis information, but there exists diagnosis information from the baseline to the most recent visits for everyone in the ADNI database. Also, the ADNI is the longitudinal study following all participants over multiple years. Therefore, the diagnosis will change over time. So, selection of diagnosis will totally depend on your scientific hypotheses and it is hard to say any uniform criteria for selecting the diagnosis. If you are interested in the most recent diagnosis, please look at Phase, Visit, and Exam Date which can tell you which diagnosis is the most recent for each individual.