What is the 2009 non-persistent minimum membership doctrine?

Article 234 of the Labor Code merely requires a 20% minimum membership during the application for union registration. It does not mandate that a union must maintain the 20% minimum membership requirement all throughout its existence. However, this does not prevent another union within the same company from challenging the status of the union as the legitimate labor organization authorized to represent the interests of the employees with the management. (G.R. No. 183317; December 21, 2009)