Thanks, whatever you can let me know will be helpful.
I copied the customers table and created a screen which shows me Name, Company Name, Full Name, and First Name so that I can make sure I am seeing the data correctly.
For every entry that I imported, I have one record where Name = Company Name = Full Name. This is fine, it is what I expect. In the First Name field I see the first name of my contact, which is also fine.
Then, for every entry that I imported, I also have one record for each opportunity. These imported so that Name = Opportunity ID, Comany Name = Company Name (as it should), and Full Name = Concatenated Company Name and Name field, eg Company Name:Name.