To me, some kind of re-publishing of non-indexed studies needs to be done somehow. I myself search theses and dissertations when I have to really get deep in a topic - these can have great data on new measures. Manual search of conference proceedings can get you info on emerging topics, and show who is working on these. Increasingly, these are popping up in e-searches. Theses and dissertations are indexed, but they are kind of out-of-the-mainstream so do not get found. I am currently doing a review, and I am including decent dissertation evidence, with the caveat that this source is not "vetted" by peer-review (although the quality was probably scrutinized highly). This is kind of "grey market" for knowledge, and it would be great to have some improved way to access these "grey market" research pubs.

Another reason for duplicate publication is for pharma to more quickly create their own snowball effect of evidence for marketing reasons - if pharma reps can walk into the doc's office with more than one article as evidence of efficacy, it gives the impression that the new drug has got something goin' on. Generate some buzz for the (patented) med.
This blogger gets credit for noting this specific example:

http://clinpsyc.blogspot.com/2008/09/cymbalta-schatz-storm-duplicate.html
If you want, read the blog and/or look at the following bits from these two articles:

Fig 3 in the J Clin Psychiatry

Fig 2 in the J Psychiatr Res

Perahia DG, Quail D, Desaiah D, Corruble E, Fava M. Switching to duloxetine
from selective serotonin reuptake inhibitor antidepressants: a multicenter trial
comparing 2 switching techniques. J Clin Psychiatry. 2008 Jan;69(1):95-105.
PubMed PMID: 18312043.

Perahia DG, Quail D, Desaiah D, Montejo AL, Schatzberg AF. Switching to
duloxetine in selective serotonin reuptake inhibitor non- and partial-responders:
effects on painful physical symptoms of depression. J Psychiatr Res. 2009
Feb;43(5):512-8. Epub 2008 Aug 15. PubMed PMID: 18707693.

That will pad your CV, but in this case it seems likely the motivation was to generate product 'buzz.'