This issue can occur if a User has two client records opened at the same time and creates a Visit Plan on each record before saving.
There are checks in place within CallConfirmLive! to prevent you from assigning a carer to two different visits at the same time however if two visit plans have been created but not yet saved, this breaks the core functionality of the system.
Please ensure that your users create a visit plan on one client record and save this before creating a visit plan on another client record.
If you require information on how to create a visit plan, please follow the steps in this Knowledge Article.