Originally Posted by
IAN-UK
My understanding was that IATA got around the limitation of two-letter codes by allowing duplication only with geographical separation (OM was for a time attributed to both Mongolian and Monarch) then encouraging use of three-letter codes.
There are a number of factors that go into controlled duplicate assignment, including geographical separation, type of operation (cargo/passenger, scheduled/charter, etc...) but in the end, basic common sense usually prevails. I've successfully petitioned IATA for a controlled duplicate alpha-alpha code assignment as recently as 2012 and it was a straightforward process if you have the proper arguments to make.