11gR2: Bug causes RMAN to remove spfile during database upgrade

The title of MOS document "Upgrade Of Database From 11.2.0.1 To 11.2.0.4 Caused RMAN Duplication To Error On Spfile" from article ID 1951266.1 tells us about a bug which needs to be avoided during 11gR2 upgrades with either a workaround or one-off patch.