The act of talking to somebody about the same thing over and over [duplicate]

It's called nagging if it's really annoying.

Best use the simple past tense though if the subordinate sentence is in the past simple.

Mary nagged at John till he finally succumbed and bought it for her.

Examples

She's been nagging about it since this morning
He's always nagging at her for staying out late

From OALD the phrasal verb to harp on about sth is one possibility, but it is informal.

• Mary's been harping on about it to John till he finally bought it for her. (informal, disapproving)

This other one, the phrasal verb to bang on (aboutsth), used in British English only, (OALD) is informal too.

• Mary's been banging on about it to John till he finally bought it for her. (informal)