Comment 4 for bug 718765

seems to be a the standard behaviour of 'metaflac --add-replay-gain': http://flac.sourceforge.net/documentation_tools_metaflac.html
> Existing ReplayGain tags will be replaced.

It's still annoying if processing large colelctiobns of files using qtgain, so I added a feature request: https://sourceforge.net/p/flac/feature-requests/107/