I'm pretty sure it's hard coded to flag a char when their desc matches one in the archive that's automatically generated somehow.

I recently had a char that was told to change his name (it was Chunth). The next login after I did, I got -1000 IMM xp for reusing a desc. After I explained in a note that the cause was probably the name change, I got the 1000 IMM xp restored. The code thought I reused Chunth's desc because it matched a character of a different name though it was the same character.

Anyway, good bug exploit I discovered: Get penalized like that and get it reversed to get +% to skills for the IMM xp to reverse the penalty, then repeat. (For the 1-2 IMMs without a sense of humor, I was joking about exploiting this bug).