Possibly UK and US English speakers will have different opinions about this - I'm from the UK. This discussion of "different from" and "different than" usages might be interesting, if not directly relevant to your question.
Twice is a numeric comparator, we say
twice as many
twice than many
So I prefer
It turns out that someone in the high risk group has twice the risk as those in the low risk group
using "than" feels completely wrong to me. However I'm not quite comfortable with that phrasing. My first problem is that someone is singular and those is plural. Secondly, and this may well just be my preference, we have a has in the first clause, so I feel that we need a verb in the second.
It turns out that someone in the high risk group has twice the risk as does someone in the low risk group