If it's something I can fix myself, fix it. If it's just reaching end-of-life I start looking for a new (or new-to-me) one and look a bit shabby until I find it.

Ties I would probably have more than one if I needed any; they feel more like a clothing item where the duplicates allow time to wash and dry (or dryclean? I have no idea how to care for a tie).

A good leather belt can easily last a decade, even a cheap PU "leather" one can last a year in a shabby state, which is fine with me while I slow-shop for a replacement. But if your lifestyle would make it more likely to get damaged and need frequent replacing, a backup so you're not left without makes more sense than taking that risk.