CSV import error: invalid project key although key is correct

[Briana]

Hello!

I'm migrating values from an old custom field to a new one by exporting the issues to csv, and then re-importing and mapping the old field to the new one. It has worked for most of my issues, however on a few projects I am getting the following error that stops the import: 

"Invalid project key: Project key or name is already used. If you want to import to an existing project the name and the key must match it exactly."

I don't understand how this is possible, since I just exported these same issues and did not change the key or project name in the file. 

I'm on JIRA Data Center version 7.2.8 with version 7.1.25 of the import wizard