It should have printed the first DATECHEM for each, not the last. Make sure you sorted properly. Or better, yet, just drop DATECHEM. If you spot check the numbers, they should be 100% correct.
This approach isn't intended to select a single observation. It will select the first nonmissing CREAT, the first nonmissing AST, the first nonmissing ALKP, etc. for each ID. They could easily be selected from different incoming observations.
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