Complex sentence: In 1991, she was a guest star in a first season "The Flash" episode as Dr. Rebecca Frost, and in 2006, she was also a guest star in "Dexter" as Dexter's first victim.
Split into simpler sentences: In 1991, she was a guest star in a first season "The Flash" episode as Dr. Rebecca Frost. In 2006, she was also a guest star in "Dexter" as Dexter's first victim.