Duplicate tag definition. breath is older and has far more posts than breathing.
Edit: NWF Renim proposed that the definition of breathing be changed to apply to animated posts where a character is breathing heavily. The update request has been edited accordingly.
I considered making it an alias, but thought there might be too many false positives that way. People might tag breathing when a character is panting without any visible condensation, for example. How about first doing the mass update and then making breathing an alias of heavy_breathing?
Edit: I'm not strictly against aliasing to breath, if you and/or other users think that would be better.