I'm getting 2 minutes and 29.98 seconds, with my 2017 MacBook Pro with 8 cores + 16 GiB ram. I'm using GNU tools, could you share more details on how you're getting 12 seconds?

    $ time awk 'BEGIN{for(i=0; i<3100000; i+=1) for(y=0; y<10; y+= 1) print i "@gmail.com"}' > email30m.sorted
    awk  > email30m.sorted  8.29s user 0.82s system 97% cpu 9.352 total

    $ ls -lh email30m.sorted
    522M Mar  8 17:14 email30m

I didn't have your email30m dataset, so I fabricated one, mine is smaller at 522M and has less entropy, since 'm assuming everyone uses gmail & uses numbers for usernames. :)

    $ time shuf email30m.sorted > email30m
    shuf email30m.sorted > email30m  15.58s user 1.69s system 98% cpu 17.561 total

I also shuffled the dataset, because that would be cheating.

    $ wc -l email30m
    31000000 email30m

    $ time sort email30m | uniq > email30m.uniq
    sort email30m  873.55s user 4.29s system 585% cpu 2:29.98 total
    uniq > email30m.uniq  4.10s user 0.34s system 2% cpu 2:29.98 total

    $ wc -l email30m.uniq
    3100000 email30m.uniq

I followed what you did, but it took way longer when running on my machine.

    $ time sort email30m.sorted | uniq > email30m.sorted.uniq
    sort email30m.sorted  413.75s user 3.66s system 468% cpu 1:29.04 total
    uniq > email30m.sorted.uniq  4.11s user 0.40s system 5% cpu 1:29.04 total

And just out of curiosity, it takes 1 minute 29 seconds to sort & uniq the presorted dataset.