Richard wonders why merge does what it does when you duplicate a
merge key in one file. See results from the following.
clear
set obs 3
input sid x
1 10
2 20
3 30
sort sid
tempfile one
save `one'
clear
set obs 4
input sid zip
1 11111
2 22222
3 33333
3 33333
sort sid
tempfile two
save `two'
use `one'
merge sid using `two'
tab _merge
list
It is generally a good idea to use options 'unique', 'uniqmaster',
'uniqusing' when you can assert that merge keys should be unique in
one or both files.
