か～か

か～か puts a か after each of two choices: "A or B". It follows a noun, or the plain form of a verb or an adjective, and the sentence then asks which one or says which one is not known yet.

"Whether or not"

With the same verb twice, the second in the ない-form, か～か means "whether or not". The whole thing goes before a verb like わかりません分かりません, きめてください決めてください or おしえてください教えてください.

One か or two

For a plain "A or B" inside a sentence, one か between the nouns is enough: こうひいコーヒーかおちゃお茶をのみます飲みます. The second か closes off the pair, so the pair can stand on its own before どちら, or before a verb like 分かりません.