Difference between 同じ and 同じく(い)


For example:

あっ！お金欲しいな！

A: 同じです (I said)

B: 同じくです (A native Japanese speaker said)

The 終止形 of 同じく is 同じい, but it seems they have the same meaning

Is there any difference between them, anything makes A and B sound different?




In that context, 同じです and 同じくです work the same, though the former is more
common. But that does not mean they are always interchangeable.

同じ and 同じく are not regular adjectives. There is no such an i-adjective as
同じい*, and 同じく is a fixed adverb meaning "likewise" or "similarly". 同じく is
typically used when repeating the same opinion or in place of the same word.

*Actually a large monolingual dictionary has an entry for 同じい as an i-adjective, but don't use it in modern standard Japanese. 同じく is the only allowed form.

  * A「私は反対です。」B「同じく私も反対です。」  
A: "I disagree." B: "I disagree, too."

  * A「大阪から来た田中です。」B「同じく佐藤です。」  
A: "I am Tanaka from Osaka. B: "I am Sato, also from Osaka."

同じくです is not very common, but it's "Same here" or "Ditto". Thus you cannot use
同じく in the following example:

  * A「XとYは違いますか？」B「同じです。」  
A: "Are X and Y different?" B: "They're the same."

