Emphasizes that something happens on every occasion or repeatedly without exception. Slightly formal or written, but can be used in speech for emphasis.
He is late every single time.
Thank you for your continued support (literally: for always taking care of me).
Uncommon spelling without the repetition mark; 毎々 is preferred.
Reduplication of 毎 (まい, 'every') with the repetition mark 々, yielding an emphatic adverbial meaning 'each and every time'.