adverb, noun which may take the genitive case particle 'no'
day after day; every single day
Emphasizes the repetitive, continuous nature of an action or state happening daily, often with a nuance of monotony or persistence.
毎日毎日、同じことの繰り返しだ。
Day after day, it's the same thing over and over.
毎日毎日の勉強が実を結んだ。
The daily, day-after-day studying paid off.