In my family tree, there are multiple slave owners that sold slaves multiple times. This was common for young children, or those who bucked the system, or if the owner died. So if you create “Sarah Johnson” age 3 because she was born on the Johnson plantation; then she is sold to punish her mother, so she winds up on the Smith plantation in a different state. Now she is 10, and hired out to the Landry family. Mr. Smith, the owner, dies, and Sarah, now 15, is sold to the Walker family, in yet another state. She is given to the teenage son, and bears several children. At any given point, if you don’t know the whole story, she might be created under several last names.
How do you prevent duplicates?
While this example might seem extreme, it is not that uncommon. If the slave was disabled, disfigured, or unable to work a full day for any reason, they might be sold quite often.