When we think of mobile telephones we mostly think in terms of them of having a single International Mobile station Equipment Identity number. For two decades, whether for an analogue or digital mobile 'phone, we have often associated, to assist in showing importance of, and drawing anology about, that mobile 'phone serial numbers are alike to vehicle chassis numbers - in essence IMEIs are intended to be unique numbers.
Having reviewed the Standards and other documentation and found no definitive statement about the requirement for a dual IMEI numbering scheme, I put out some enquiries and I am grateful to those who assisted. I am told it would appear the occurrence of the dual IMEIs are due to there being two radio chipsets in some handsets. As I understand it, also, I won't find anything in the Standards about this matter - just yet. Moreover, it does not automatically follow that a handset having Dual SIM/USIM slots implies or infers that the handset has two radio chipsets, thus two IMEIs.
Evidentially, of course it is noted this matter impacts in numerous ways when conducting examination using automated physical and/or logical harvesting of data and the much-needed handset (manual) examination.