Yes, it's normal for autistic adults to talk to themselves occasionally. Many people on the autism spectrum like to review conversations to themselves for numerous reasons.
This can include repeating lines from their favorite TV shows or movies, and this is known as "scripting". It's a repetitive behavior that's common amongst autistic people, and can be a source of comfort if the person has anxiety or excitement about an event that occurred recently.
Adam Winsler, an associate professor of psychology at George Mason University, said "parents should not worry when their pre-schoolers talk to themselves. In fact, they should encourage it."
Winsler conducted the first-ever study which looked at private speech in autistic children, which was published in Early Childhood Research Quarterly. The study found that high-functioning autistic children talk to themselves often and in the same ways that non-autistic children do. It also showed that 5-year-olds do better on motor tasks when they talk to themselves out loud than when they are silent.
A person with autism has good vocabulary but unusual ways of expressing themselves. Usually, they will talk in a monotone voice and won't recognize the need to control the volume of their voice. For example, someone with autism might speak loudly in a movie theater without realizing that it's a place to speak quietly in.
Autistic kids with high-functioning autism (HFA) and asperger's syndrome usually means they are obsessed with a certain topic and will frequently speak about it. Children with autism may constantly talk about cartoon characters, movies, race cars, and other things they are familiar with.