NPCs with dialogue options (such as city guards) play their welcome voiceline every time you select an option. For example, talking to a bluffwatcher (Thunderbluff guard) causes them to say "What are you looking for?" when you first talk to them, and again when you select "a profession trainer", and again when you select a profession. Ideally the option tree would be voiced, but if that's not possible then just having them not repeat would be an improvement