st: merge creates duplicates in master data

[Will Hauser <whauseriii@gmail.com>]

Hello all,

I am experiencing unexpected behavior in Stata 10 when using the merge 
command.
I am matching two lists based on a series of string variables (first 
name, last name, initials) and one numeric region identifier.  I have 
carefully cleaned the string variables of excess spaces and punctuation 
marks but they are inherently difficult to match as the name on one list 
may correspond to a nick name or abbreviation on the other (e.g. 
"WILLIAM" may correspond with "W" or "BILL").  My approach to this 
problem is to make multiple merges between the two lists each time using 
less information.  For example, the first merge uses first name, last 
name, and region.  The second uses first initial, last name, and 
region.  The third just last name and region (and so on).  Since the 
master data is inviolate subsequent mismatches should never overwrite 
earlier 'good' matches.  I am using the update option but not the 
replace option.  I am not using the unique option since the variables do 
not uniquely identify the cases in either the master or the using.

From what I can tell Stata is duplicating cases in the master dataset.  
The end result is 10 pairs of duplicate entries that appear identical in 
every way save for the _merge summary variable from the last merge.  The 
summary variable indicates using agrees with master (3) for one of the 
duplicates and indicates that using does not agree with master for the 
other (5).  There are no missing values in either list and I can see 
nothing special about the entries that are duplicated.  I have used the 
duplicates command to verify that these duplicates are not present in 
the master data prior to merging.

I assume this is not a bug but is rather something about the merge 
command I am misunderstanding and that concerns me very much.  I would 
be happy to provide the lists and the relevant portion of the do file if 
anyone is interested.  The lists are public and are not unusually long 
(958 cases in the master and 593 cases in the using).

Thanks for your insight,

William Hauser
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