🐛 Fixed duplicate error columns in members import error CSV
🐛 Fixed duplicate error columns in members import error CSV

fixes https://github.com/TryGhost/Team/issues/1911

Kept on creating a new error column when unparsing the error list. So for every error we would create a new column when it was already added.

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost