So my last riddle turned out to be too easy. Lets see if this one is difficult enough.
You wake up with a huge headache and look around you. Somehow you managed to end up in a white room of about 2 by 2 meters with no furniture or windows. The only thing that stands out is a white door with a small computer screen next to it. You try opening the door but its locked. It doesn't seem like it can be forced either. On the computer screen you see that the door is locked with a six-digit numeric passcode. Fortunately whoever set the passcode put a hint with it, probably because they aren't clever enough to remember the passcode themselves. The hint reads:
2351389233159728657, I shall unite with my former self, but once united I cannot be split. Tell me, what will I be next?
Which code do you enter to open the door?