Despite I love GMail with all its features, it seems that I should consider using a desktop software email client like Thunderbird or Evolution in my current conditions.
So I'd like to optimize my GMail layout for desktop IMAP client fiendlyness. That's why I'd like to ensure all my messages have one and only one label applied (because with IMAP labels appear as folders and multilabelling results in message dupes).
So, how to find all messages, having more than one label applied?