How can you tell it’s the familiar form
why not "Has tenido una coneja?"
YOU in spanish can be either tú or ustedes, so "¿Han tenido un conejo?" should be accepted also. "Have USTEDES had a rabbit?
You're right! You should report it.
Not to mention usted, singular. It seems DL only likes the familiar. Maybe only real friends can discuss their rabbits together. For usted acquaintances, they'll just have to discuss dogs, cats, and penguins. ;-)
Has habido un conejo - what is wrong with using haber?