Happens on new files never imported and on files I've imported before (creating new events the 2nd time and unchecking "do not import duplicates")
Why are you "unchecking" the "Do not import duplicates", if you do not want to import duplicates?
Which of the two files are you seeing highlighted, when you use "File > Reveal in Finder", when you select one of the image versions with duplicates? It should be the "DC101010 (1).jpg"
The only way, i can reproduce your issue when importing referenced images, is when I use the Import setting
"Store Files" with the folder selected, where the photos to be imported are currently in and the "Copy files" option selected.
This will write a duplicate to the folder you are importing from, in the example above in the Pictures folder.
I unchecked the box for preventing duplicates just to try things out. It did appear to not affect the issue either way. It is checked again now. When I reveal in Finder, the first file is the one selected. I ran a test and the duplicate files appeared on the first import. I will play with the checkbox you pointed out and see if it causes any differences. Thanks for looking into this.
Resolution: Change the 'Backup Location: Backup To' setting to 'None'
Backup Location: This feature allows you to send your master files to two separate locations during the import process. Once you enable Backup Location, you need to choose which connected hard drive will be used for the second set of masters. This is a nice feature for those who want to erase their memory cards right after import.