Suppose Ammar finished at position $k$. Then there are $k-1$ runners that finished before him and $2k-2$ runners that have finished after him, for a total of $3k-2$ runners. Julia finished at position $k+10$. There are $2k-12$ runners that finished after her, and $k+9$ runners that finished before her. Since the number of runners that finished before her is twice of the number of runners that finished after her, then $k+9 = 2(2k-12)$, which yields $k=11$. So there are $3 \cdot 11 - 2 = \boxed{31}$ runners.