That isn't a bug. That is what happens when you overkill someone with cold damage. First they get frozen solid (first death message) then they explode a few seconds later (second death message). Perhaps they could remove the second one to avoid confusion.
Side note: I usually try not to overkill things with cold damage as it destroys treasure other than gold just like petrification or disintegrate.
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Side note: I usually try not to overkill things with cold damage as it destroys treasure other than gold just like petrification or disintegrate.