My error was being caused because I had to upload the same products several times. My process was to drop the xcart_products table, then recreate it, and then empty the xcart_thumbnails table. xcart support informed me that I also needed to empty the xcart_pricing table, which solved the problem.
I was not simply checking the 'drop all products before import' option because I need to control the auto_incremented productid number. By resetting it to 1, the images and options data does not have to be re-entered in their respective tables.
Now, I am struggling with implementing offline processing for my client. As noted at
http://forum.x-cart.com/viewtopic.php?t=862&highlight=pgp PGP does not seem to work. I have reset my config.php to store credit card numbers, but I expect to be working on this problem for days.
My client's site is
http://www.nylashoes.com/store/.