bluff01 Phone is encrypted with password -> if it is reseted with duress password will my data not be encrypted anymore?
At the risk of oversimplifying: in normal operation, your information is encrypted with a key known by only one party, the security chip inside the phone. When you present your PIN/password/passphrase, the security chip makes the encryption key available to the hardware accelerator that encrypts and decrypts storage blocks. If the duress function is triggered, the security chip quickly and permanently forgets the encryption key, so now your information is encrypted with a key that nobody knows. In theory your information can be recovered, but in theory it would take so long to guess the data encryption key that most people needn't worry about it. There are many details, but I believe this is the core.
bluff01 And is it even useful to activate duress pw if i am using a very long password to encrypt my graphene?
The idea of a duress password is that no matter how long your password is maybe somebody can threaten you to type it in, and maybe you would prefer to have the information vanish instead. In some legal jurisdictions "destruction of data" may be a crime, so this may not be a good idea. Also, it is possible that somebody trying to coerce you into unlocking your phone would become angry if your information vanished, and take revenge.
Whether or not to activate the duress function depends on one's personal circumstances.