Please try the below code as well,it is slightly different from the existing posts
data have;
input ID Product $ Days;
datalines;
100 A 1
101 B 2
102 A 1
102 B 1
102 C 1
103 A 5
103 A 5
;
data want;
set have;
by id;
retain new;
if first.id then new=product;
else new=catx('',new,product);
if last.id;
run;
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