1) If you used a cell phone to register, IP addresses are rotated when using a cell, so someone else in your area is sad too.
If you didn't use a cell phone to register and used a computer it means:
2) Someone in your residence knows you used the site and created an account to keep tabs on you, not expecting the mods would reach out to you with a "warning regarding alt accounts."
3) Someone in your residence is also suicidal
4) If you are alone then your suicidal neighbor is hacking your WiFi
5) You have malware and someone gained access to your machine and then registered using your IP (this is easier for a hacker to do than people realize and often involves social engineering rather than really extreme programming feats).
6) Your home Internet has a rotating IP and someone in your area is also suicidal.
It is most likely your cell phone has a rotating IP and you used that to register, however the website creators may be tracking things like user agent, canvas fingerprint, font metrics, etc, and may have concluded that not only was the IP address the same, but also the browser was the same, which would mean someone using the same computer as you and registering again. For more about browser fingerprinting, go to coveryourtracks.eff.org.
If the mod messaged you and wrote "Do you have an alternate account or did someone find your sasu account and try to register another account? You should know they are on to you!" it wouldn't look good. Also, this is much much more likely a coincidence based on rotating IP addresses. On rare occasion browser fingerprints are the same.
