fix department can be apply added repeatedly bu

app/services/create_add_department_apply_service.rb

@@ -14,6 +14,7 @@ class CreateAddDepartmentApplyService < ApplicationService
 
     school = School.find_by(id: params[:school_id])
     raise Error, '学校/单位不存在' if school.blank?
+    raise Error, '部门已存在' if school.departments.exists?(name: name)
 
     department = Department.new
     department.name   = name