For some reason, I have duplicate category fields for all my issues under one project. When I go to project settings -> Issues Types I only have selected one custom field for the category but when I check the project in the list view, the category is added twice without being able to unselect one, it's either both or none.
https://monosnap.com/file/35NnyzyebdF4OullQrKHrLWGI5Gwtv -> Here is a short video to better understand the issue.
maybe you've got confused with system and custom fields?
"category" is a system field in order to select each project category/ group so your team can view work across related projects in one place - see https://support.atlassian.com/jira-cloud-administration/docs/add-assign-and-delete-project-categories/
Of course, you can create a custom field with the same name which I highly recommend to avoid because searching for issues will get complicated.