st: Identify and delete duplicate obs

[Rongrong Zhang <r05zhang@gmail.com>]

Hello,

my dataset has the following structure

industrynumber   naics
1000                     .
1001                      114
100101                  114
100102                   114
........

both variables are string .
the first observation has a missing value for naics. observations
sharing the same four digit (e.g. 1001) will have the same naics, I
would like to keep one observation only for the same naics

i used
bysort naics: gen dup=cond(_N==1, 0, _n)

this command will count missing naics as well, I have thousands of
missing naics records, in which case, dup is a large number.

how should I replace dup value when the observation is missing naics??

this did not work"replace  dup=-1 if naics==" "

thank you,
Rochelle
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