ECDSA with common nonce? I'm new to cryptography, and I've searching about ECDSA because I'm trying to solve a CTF. 

I've already check this site and Google, and I think I'm in the right path, but probably I am missing something.

I have access to messages and the ECDSA signature that they generate. The format is always the same:

Example of a message:

{"session_id": "6621a96c7db568374f2885d6d135f395010e75a94ec2233a433ff8e2", "user": peter}


And the signature have always the same first half.

znnlVaDhCokfqzU5figrY2cZ1nk87rH/

Example of two signatures: 

znnlVaDhCokfqzU5figrY2cZ1nk87rH/+Zc/DvAEIyjZ4pv8SVmCsWLtq+yJrtFJ 
znnlVaDhCokfqzU5figrY2cZ1nk87rH/zcCHDV2rLJ6nhdjE9vzblfpkzrhqzVjY 

This looks like signatures generated with NIST192p curve (because of the size) ?

There are similar CTF that used the same thing as this one, and a solution like this one: http://ropnroll.co.uk/2017/05/breaking-ecdsa/

But the new generated signature doesn't start with the same part as the above ones.

They seem to be based on this one, https://antonio-bc.blogspot.com/ (you can look for ECDSA). 

Here is again another CTF with a similar issue, but with a different curve. http://itemize.no/2016/08/26/IceCTF-contract-task/

Also I've checked the video from LiveOverflow that tackle the same issue, https://www.youtube.com/watch?v=-UcCMjQab4w

Since these are solutions for previous CTF, I'm thinking the the way I'm creating the messages isn't the same as the server. 

This ECDSA implementation should be vulnerable, right ?
This is the modified code that I have right now.
You are right. And this had been exploited in the past in the PS3 and Bitcoin wallet events. 

The first half of the signature is the x coordinate of $k \times G$, and $k$ is the ephemeral key that must be unique to each signature generated. If it's static then the following formula would recover the private signing key: 

$k = {{z_1 - z_2} \over {s_1 - s_2}}$ and $d = {{sk-z} \over r}$ where $z_1$ and $z_2$ are the truncated hash bitstring, $s_1$ and $s_2$ are the two corresponding signature component, $r$ being the fixed part in both signatures, and $d$ the private key we hunt. 

For more, Wikipedia has an article covering ECDSA and its security.