If you're asking that question you should not be posting here, but since it is late you will get an explanation. The user ID changes with each thread, and with each IP address. So every time you change IP you will be new user, and get a new ID, and have to complete the captcha. Each new thread will be a new ID regardless of your IP being the same or different.
No reason to "do double dash" at all, so just forget about that.
You should not be typing anything into the top 3 sections of the Quick Reply box, ever.
You can get absolutely flamed for asking newfag questions here, so if you want to interact in a stress free environment and ask all the dumb questions you want, spend your time at the following site:
reddit.com/r/CBTS_Stream