In SAS, how do get distinct counts of a variable that has multiple observations for each individual?

Question

A simple question, but I have three variables in my dataset: ID, ICDCode, and a Visit Date. There are multiple occurrences of each ICDCode per person(ID). How do I obtain a total, distinct count of icdcodes for the entire dataset, without counting a certain ICDCode twice for an individual? For example, I want to say that there are 100 cases of Heart Disease in my dataset (without counting heart disease 10 times for the same person).
Below is code I have tried:

    proc freq data= cases;
    table ICDCode;
    run;

    proc sql;
    select ICDCode,
    count(*) as Frequency
    from cases
    group by ID;
    quit;
    

Answer 1

How about simply: (Given that 429.9 represent heart disease)

data cases;
input ID ICDCode;
datalines;
1 429.9
1 429.9
1 2    
1 3    
3 429.9
3 429.9
3 3    
2 1    
2 2    
;

proc sql;
   select count(distinct ID) as n
   from cases
   where ICDCode = 429.9;
run;