Why did the germans encrypted twice a random three alphabet word in enigma?

Question

I have read that the Germans used to encipher a random three alphabet word chosen by the operator twice and put that before the enciphered message.

What purpose did that have and what key they were using to encipher the random three alphabet word?

Answer 1

The goal of this practice was as stated below (in bold) and this was done after the key settings were chosen (see link here):

Because the sender could choose the rotor start (i.e., key) settings, it was necessary to inform the recipient of the chosen settings or Grundstellung and this formed the first three letters of the transmitted message. These three letters were actually repeated and sent twice in early transmissions as a "double indicator" to enable the recipient to confirm they had been correctly received. This double indicator was another source of weakness in the system which was exploited by the Polish codebreakers until the Germans eventually abandoned the practice.

This is essentially a repeated plaintext and the knowledge that it was there led to some attacks, including the breakthrough by the

Polish mathematicians (Marian Rejewski, Jerzy Różycki, Henryk Zygalski) make astonishing breakthroughs in reconstructing the machine but also in diagnosing the use of a repeated three letter message indicator at the start of each message. Once again observe that this is not a known plaintext, but it is knowledge of a repeated section of plaintext. The repetition combined with permutation theory allows them to develop an attack to recover the rotor settings independently of the plugboard. They develop manual and electromechanical tools to implement the attack. Their methods struggled to scale when the German increased the number of possible rotors from three to five in 1938.

The quote above is from this question's answer here