There are several synonyms for the phrase "harped up on," which refers to someone repeatedly bringing up a particular topic or issue in conversation. Some similar phrases include "beating a dead horse," "flogging a dead horse," "drilling it into someone's head," and "repeating like a broken record." These phrases all express a sense of annoyance or frustration with the persistence of the speaker's focus on a particular subject. Other related expressions include "obsessed with," "fixated on," and "can't let go of," which all suggest a preoccupation with a particular idea or issue.