χαίρετε
How frequently does the dual occur, and does it warrant memorisation? Thus far I have not done so because I had studied Attic as a supplement to Koine.
Χαίρε Skirnir
My teachers, when i started learning Greek at the University, told us not to bother much learning those forms because they are not very frequent in texts and not compulsory ; and anyways they are very easily acknowledgeable.
And indeed, i’ve been learning then translating Greek texts for over a decade now and came accros those dual forms maybe twice or thrice 
.
Yet, i recommend you have a close look at them, just to see what they look like and so it rings a bell when you come across it in a text. There are hints anyways, either you are dealing with the description of a body (in which case you will certainly have two ears, two legs, two arms, etc.) or of two characters who usually go along two by two (Castor & Pollux, Achilles & Patroclus…).
It depends on what you are reading. It’s not uncommon in Homer. It’s less common, but still occurs, in classical Attic (I heard once that using it was characteristic of the speech of aristocrats) and pretty much non-existant in Koine. Later learnèd writers will from time to time perform a little necromancy and use it.
annis, I love that necromancy reference. 
Even though it’s rare, isn’t it so easy a system to learn?
compared to the sg. of the first declension where you have ᾱ (after ρ,ι,ε)/η and ᾰ, you always have -ᾱ for
N./A./V. and -αιν for G./D., in the second declension -ω and -οιν respectively, and in the consonant
declensions -ε and -οιν nearly throughout.
in verbs, primary tenses have the same for 2nd and 3rd persons, (thematic vowel+)-τον/-σθον
(1st dual uses the 1st pl.) for the indicative/subj; 2nd -τον/-σθον, 3rd -των/-σθων for the imperative.
in secondary tenses, 3rd person changes to long vowel (thematic vowel+)-την/-σθην.
My teacher had never taught us the dual but he glanced over it when we did meet one instance in Plato’s
Apology where Socrates speaks of Callias, son of Hipponicus, and the education of his two sons.
hi, it would also be useful to learn specifically the duals of the pronouns, which can look quite different, e.g. the dual of the personal pronoun (in red):
aristophanes aves lines 13-15:
“ἦ δεινὰ νὼ δέδρακεν οὑκ τῶν ὀρνέων,
ὁ πινακοπώλης φιλοκράτης μελαγχολῶν,
ὃς τώδ᾽ ἔφασκε νῷν φράσειν τὸν Τηρέα”
cheers, chad 