Checking for Duplicate Ids | SAS Code Fragments

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data ids;
  input id;
cards;
1
2
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9
;
run;

proc sort data=ids out=ids2;
  by id;
run;

data dupes;
  set ids2;
  by id;
  if not (first.id and last.id) then output;
run;

proc print data=dupes;
run;