st: Problems with expand og reverting to original dataset

[Grethe Søndergaard <dkstatstata@gmail.com>]

Hello

My dataset consists of siblings (two siblings from each family).

person_id1
mother_id
father_id
sibling_id
birth date


I only want to keep siblings in my dataset with a certain age
difference, lets say 5 years or less.
I have tried to do this using expand but when I am not sure how to
revert to my original dataset. This is what I have done:

** duplicating each observation [no] times
expand no, gen(dub)

** creating an (arbitary) id to keep track of the duplicates
sort mother_id father_id person_id1
by mother_id father_id person_id1: gen copyid = _n

** copying
sort mother_id father_id
by mother_id father_id: gen person_id2 = person_id1[no*copyid]

** adding age difference variable
by mother_id father_id: gen birthdate_2  = birthdate_1[no*copyid]

** deleting redundant copies
drop if person_id1 >= person_id2

* creating age differences
gen agedif = birthdate_1 - birthdate_2
replace agedif=agedif *-1 if agedif<0
keep if agedif<6

So far so good. My dataset now consits of pairs of siblings with an
age difference of less than 6 years. There are no dublicates. But how
do I revert to my original dataset??
(I have tried to keep if dub==0 but that deleted a lot of observations
- and that shouldn't happen since there were no dublicates)

I hope somebody can help since I am stuck with this problem.
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