st: how to assign new identifier numbers with duplicates

["Linda, X Zhu" <lx_zhu@hotmail.com>]

Dear Stata users,

I am using a firm-level panel data for performance analysis of firms. But I 
found that my dataset has duplicated identifiers (e.g. the same identity 
numbers for two firms in two different regions in a certain year). My 
dataset looks like as follows (a1 is a code for high-level region, a2 is the 
subregions, a3 is the firm identity number):

year	a1	a2	a3
1995	450	57	206141
1995	450	54	206141
1996	450	57	206141
1996	450	54	206141
1997	450	57	206141
1997	450	54	206141
1995	470	41	223243
1995	470	43	223243
1995	470	44	223243
1996	470	41	223243
1996	470	44	223243
1997	470	41	223243
1998	470	41	223243
2000	470	41	223243


This moment, I don�t want to consider the differences of subregions. So, I 
want to change the identity number such that I have uniquely identified 
observations by the identifier variable a3 and year. From the help, I found 
and did as follows (a3 is my identifier variable):

duplicates report a3 year

Duplicates in terms of a3 year

--------------------------------------
  copies | observations       surplus
----------+---------------------------
       1 |        14906             0
       2 |         2176          1088
       3 |           87            58
--------------------------------------

. duplicates tag a3 year, gen(isdup)

Duplicates in terms of a3 year

. duplicates tag a3 year, gen(isdup)

Duplicates in terms of a3 year

. edit if isdup

This gives me a way to reassign an identity number for each duplicated firm. 
But since my dataset is big (I need to renumber 1088+58=1146 firms), I think 
it would be more convenient to use other commands, e.g. replace. So, I did 
the following:

. replace a3=1000000+a3 if isdup==1
(2176 real changes made)
. replace a3=2000000+a3 if isdup==2
(87 real changes made)

But then I changed all the duplicated numbers into another (still the same) 
number for two or three firms. Could somebody tell me how to change identity 
numbers in an easier way?

Thanks for your kind response.

Linda
Wageningen University NL

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