If a language has it's own number of plural forms decreased, previously distinct TranslationMessages would now appear the same.
Example: a language might have 2 pluralforms, and two different translation messages A and B for a single potmsgset (eg. A.msgstr0=1, A.msgstr1=2 and B.msgstr0=1, B.msgstr1=3). When we reduce the number of plural forms for a language to 1, when trying to update this translation, a selectOne in _findTranslationMessage would return two messages instead of only one.
Currently, we have a problem with 17 messages like this in our database, and they are all for 1 plural form languages. See https://pastebin.canonical.com/2812/
There are different approaches to fixing this: fix the existing data, and make sure such messages are cleared up when the number of pluralforms for a language is reduced, or use all 4/6 pluralforms for finding matching translationmessages (which might make it impossible to activate existing translation, eg. an imported one, if it had more plural forms on import, but would instead create a new message).
What exactly failed: https:/