clear all
set obs 2
gen x="a" in 1
replace x="b" in 2
save temp1
clear
set obs 4
gen y=_n*10
gen id=_n
save temp2
use temp1, clear
gen id=1
append using temp1
replace id=2 if id==.
append using temp1
replace id=3 if id==.
append using temp1
replace id=4 if id==.
merge m:1 id using temp2
erase temp1.dta
erase temp2.dta
sort x id
list