st: Duplicate observations

[emanuele mazzini <madsoenistata@gmail.com>]

Hello to everybody,

I have an issue about duplicate observations that I find puzzling to solve.
I have data on country-pairs by year and I am interested in two
specific variables, a date and, say a variable which I call x_1.

Specifically, my data look like this :

reporter  partner   year       date         x_1

Albania  Austria   1980   6dec1980     n_1
Albania  Austria   1980  15nov1980    n_1
.         .        .
.         .        .
.         .        .

As you may have noticed observations differ amongst them only by date
and I need to drop them so as to keep the most recent one (hence, in
this case the second one).

I ran the following commands:

duplicates tag reporter partner year, generate(dup)

by reporter partner year (x_1 -date), sort: gen duplicates=_n

so as to be able to identify duplicates and then - among those with
dup >0 - drop those for which duplicates > 1.
This method was suggested in this thread (I take this opportunity to
thank again), but it seems not to work for some observations.
Take, for instance the following example:

reporter partner    year      date         x_1    dup     duplicates
Albania Germany 1967 08apr1967    n_1      1           1
Albania Germany 1967 17dec1967   n_1      1           2

As you may notice, Stata identifies the observation occurred the
17dec1967 as those with duplicates > 1 (which will then be dropped),
while I would have expected Stata to make the opposite.

Can anyone explain me why and, possibly, tell me how to deal with such issue?

Thank you very much in advance,

Emanuele
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