Question on rationale behind doubled "Int. Math. Res. Not. IMRN" journal name I am not sure this forum is the right place for this question, but I'm pretty sure it would quickly be closed at MathOverflow.

There is a well-known mathematics journal called "International Mathematics Research Notices", published by OUP.  It's almost invariably cited as "Int. Math. Res. Not. IMRN" [sic] or similar, which has always struck me as oddly redundant.  The curious ubiquity of the abbreviation is doubtless due to the fact that MathSciNet provides references in that format: "JOURNAL = {Int. Math. Res. Not. IMRN}".

On the other hand, Worldcat has many listings for the journal, some of which have the double IMRN, and some of which don't.  As far as I can tell, the journal's page doesn't specify a preferred abbreviated form.  

Another possibly relevant factor is that the journal is usually referred to be this initialism: people call it "IMRN" in ordinary conversation, which isn't the case for any other journals I can think of, JAMS perhaps excepted if you count pronouncing it (and nobody writes "J. Am. Math. Soc. JAMS").

The question: why does MathSciNet double the journal name, and should I be doing the same?  (Perhaps the underlying question is: who decides on official journal abbreviations?)
This is just speculation, but the reason could be because of the slightly complicated history of the IMRN journal. 

Throughout you should remember that MathSciNet grew out of what originally was a print publication called Mathematical Reviews. It was extremely important that they have a uniform house style for referring to journals. 

Originally there was a journal called International Mathematics Research Notices published by Duke Univ. Press from 1991-2001. On MathSciNet its abbreviation was Internat. Math. Res. Notices. When the journal moved publishers to Hindawi in 2002, the abbreviation became Int. Math. Res. Not. on MathSciNet. In 2005 Hindawi started a new journal called International Mathematics Research Papers, which was indexed by MathSciNet under the abbreviation IMRP Int. Math. Res. Pap. 

In 2006/7, the two journals were both transferred to Oxford Univ. Press. There was some relationship between the two journals (I remember seeing it in an old author submission guideline) and the two are treated as sort of a pair. Probably in an effort to clearly distinguish the two journals, either OUP or MathSciNet decided to list the two journals as, respectively, International Mathematics Research Notices. IMRN and International Mathematics Research Papers. IMRP with abbreviations Int. Math. Res. Not. IMRN and Int. Math. Res. Pap. IMRP. 

In 2009 or thereabouts, OUP merged IMRP into IMRN. 

I would surmise that the "duplication" appeared in an effort to add some redundancy for better disambiguation of two journals with extremely similar titles. Now it survives as a vestigial testament to the journal's history.