Lightroom :: Why Does LR Append -2 To File Names / When There Are No Duplicates

I have been using Lightroom since V. 1. I have noticed that many of my file names have been renamed in LR (now V5.3)  by changing the filename from "XXX_yyyy.NEF" to "XXX_yyyy-2.NEF". IF I look in the folder where the images are stored, both versions of the file are there. I went into LR and deleted the "-2" version and re-imported the original files. In some cases this worked and the LR file now has the original name. In some other cases, LR re-named on import and appended the "-2" to the file name. I checked and it turned out there were other files with the same name on the disk and in the LR catalogue, so I understand why these files were renamed. In other cases, however, there is no other file in LR with the same name. Yet, when I try to delete the "-2" file in LR and re-import the original file (as I did earlier),  LR again renames my file by appending the "-2" suffix to the file name. It does not copy the file, it actually renames the file in the folder where it is stored. I cannot understand why the file name is being changed in this case.
 
why LR is doing this and how can I prevent it?I am running LR under 64-bit Win 7 with 12 GB RAM, the problem seems to have occurred primarily with Nikon D200 RAW (.NEF) files.