Please note that this word is supposed to be pronounced like [수:건], so the first syllable is longer
As of 2012, long vowels are already obsolete in Korean, except for a few older speakers in Seoul.
Do you mean that 사건 is not pronounced [사껀] anymore? Because that is the pronounciation I see in Naver dict when I look that word up.
Now, I know this isn't the same as long vowels per se, but this was what I meant with my comment nonetheless.
I mean long vowels, not double consonants.