Is it normal to have a high hCG level with only one baby?

Possible. The values vary greatly from pt to pt and preg to preg. (see prior answer on normal values). If HCG is out of normal range for gestation then further evaluation is warranted.
Possible. The range for the HCG level at any point in pregnancy is quite wide. So two people at the same point in a pregnancy could have very different HCG levels. However, a very high HCG could signal a problem with the pregnancy.