Hi @Steven_Bennion, thanks for reaching out! As @lpb mentioned, this type of issue is usually related to recurring tasks. When you mark a recurring task complete, it will automatically duplicate and show as incomplete in your task list with a new due date. This is currently expected behaviour, you can learn more here.
Another option is that the task was created via a project template. For example, if you create a new project from a template and the template has tasks assigned to you, all these tasks will be duplicated (including due date, custom fields, etc) and will show incomplete.
If any of these options apply, I would encourage you to contact our support team and send them the URL of the task. They will be able to confirm more specific details of what happened!