+ AA6YQ comments below
There are 58 (+/-) QSO parties with county abbreviations supported by N1MM+. Those abbreviations range between 3 and 6 characters in length - four characters was for the particular QSO party Phil logged the contacts in (I'm guessing it was the Illinois QSO Party). There is a file distributed with the N1MM Logger+ program (actually with each program update) that contains lists of the abbreviations used in all of those contests, but that list does not indicate what the actual county name is, only which abbreviations are considered valid in each contest. The correspondence between the abbreviations and the full county names would presumably be found on the applicable contest sponsor web sites. The list could change from time to time depending on whether the sponsors for one of the contests choose to change their abbreviation list or format for some reason. For these reasons, I don't think this list would be useful for your suggested purpose.
+ if Jim AD1C can maintain an application that performs the county name "translations" in batch, then it should be possible to perform them interactively. I'll ask him about it.
If Phil could report the exact times logged in DXKeeper for the two QSOs, that would help pin down what is happening. If they are identical to the second, N1MM+ must be issuing <contactinfo> messages with identical times, which would be surprising given that it logs them with different times, but not impossible. If they differ by two seconds (or four or six seconds), the issue is likely with the UDP message transmission / buffering / decoding process.
In any case, my own recommendation would be for people wanting to transfer contacts from these contests to use the ADIF export / AD1C county code conversion / ADIF import procedure. Not only is it less likely to have problems, the visibility of the ADIF files makes it much easier to diagnose and debug any problems that might arise,
+ I'll add a "Log debugging info" option to the next version of the N1MM-DXKeeper Gateway so that we can better track down problems like this one.