Doppelgangers now exist, how do you authenticate yourself?

Some years back I was in a D&D campaign where doppelgangers became a major ongoing concern. It turned out that in that case doppelgangers built up their image of the person they wanted to mimic through careful observation, but thanks to the general prudishness of society doppelgangers rarely ever caught glimpses of peoples' genitals. So we ultimately came up with the "crotch check" system. Doppelgangers usually couldn't form plausible genitalia.

Same as I do now, passwords. Unless they inherited my memories, in which case who's to say I'm not the doppelganger?
- Lol, I think passwords are very trival to get.
  I mean, they got the ability to look like you, you gotta assume they can get CCTV footage of you typing your passwords.
  
  I don't see how looking like me would help them get CCTV footage, unless it's through some sort of supernatural means. And even if they could find footage of me entering a password, they'd be hard-pressed to extract individual keystrokes from a typical CCTV quality recording. Perhaps it's technically possible, but it's certainly not trivial.

My friends would notice immediately when we started playing any fps games after work. I don't wanna toot my own horn too much, but I am decently above average at fps games and my friends tend to like having me on their team since I am almost always top fragging.
If they aren't weirdly good at shooters people are gonna get suspicious quick lol

Be too useless to merit a doppelganger. To paraphrase zhuangzi, few know the usefulness of uselessness.

I would be signing all my communications cryptographically and sharing a key in person so people can validate whether a message not delivered face to face is really from me.
- Cryptography is the only correct answer

If they only mimic me and have some knowledge based on what is on my phone/online, they probably don't speak the same foreign languages as I on the same level. So I could authenticate myself by proving that I speak those languages.

Does my doppelganger have the same tastes as me? If not, then only the real me would walk around with Amy Rose and/or Sonamy (Sonic x Amy Rose) wallpapers on all his devices without the slightest hint of shame or embarrassment.
If he has the same taste, then we'll play SoulCalibur 3 together.

Assuming i'm caught by someone that know me well, just ask me about the slight red patch on my face and tell me it's allergic.

Assuming they haven't mugged me, I'd just sign into one of my accounts with my yubikey.

So far, I've been compromised twice. The second, hijacking my phone via sim swap, means there's a doppleganger who is pretty good at being me. Luckily I was able to restore my phone to me in about 1/2 hr because I still had a landline phone at the time, and so got ahold of my provider quickly.