How do I detect duplicate cases without having an ID number?

This example syntax detects if cases have identical values jointly on three variables, a, b and c. If a case has the same three values as an earlier case in the file then the variable copy will equal 3 (or more generally the number of variables being compared).

sort cases by a.
exe.
compute copy=0.
exe.
do repeat r=a b c.
if (r eq lag(r)) copy=copy+1.
end repeat.
exe.