Preach to the converted
Definition 1 of 1
Pro Tip 1/3Common Usage
Often used when someone insists on a point that everyone already agrees with or understands. 
When the teacher kept explaining the rule, a student said, 'You're preaching to the converted.'
Pro Tip 2/3Conversational Tone
Mainly used in casual or informal conversations, not in formal writing. 
At the family dinner, Uncle Joe was preaching to the converted about the benefits of organic food.
Pro Tip 3/3Slight Frustration
Can express mild irritation or frustration at having to hear something already accepted. 
We all recycle here; you're preaching to the converted.