Learn Turkish: Kendi kendine konuşan insan deli midir? - Is a person who talks to himself crazy?

The reduplicated phrase kendi kendine literally translates as "oneself to oneself". The dative -e ending provides the "to" direction. Because the verb konuşmak (to talk) involves directing your words to someone, kendi kendine konuşmak naturally means "to talk to oneself".

Does kendi kendine always translate to "to oneself" like it does in this sentence?

Not always! While it means "to himself" here because of the speaking context, kendi kendine frequently translates as "by itself/oneself", "spontaneously", or "independently". For example, if you said Makine kendi kendine durdu, it would mean "The machine stopped by itself."

How does the word konuşan function here instead of a tense like konuşuyor?

Konuşan uses the subject participle suffix -an (or -en), which acts like the English relative clause "who talks" or "talking". It turns the verbal phrase into an adjective that modifies insan. The entire block Kendi kendine konuşan insan acts as the subject: "A person who talks to himself".

Why does the sentence end with midir instead of just asking deli mi?