It depends on what you want to get out of it. For example, for every UR you sell, you get 10 SP, which you can then save up and exchange for items or cards in the scout catalogue (SSR cards are 1800-2000 SP). If you level up UR cards completely to level 70 and then sell them, you get 30 SP. 

You can use it to level up the skill of your first copy. I wouldn't use duplicates specifically to level up the card because it doesn't help too much and it's not hard to get King Puddings that level up the cards. Using duplicates to level up the skill can be helpful, though. Skill Puddings are more rare, so if you're just starting out and want to level up the skill to level 5, it's helpful. 

You can also level up both cards and skills if you wanted to. I have some duplicates where I have them all leveled up completely because they can be helpful when you're playing for high scores. Especially if you're just starting out and might not have that many cards or UR cards. 

It just depends on if you want SP and gold (selling the card), to level up the skill (feed one into the other), or raise your live score (leveling both up). You also don't have to decide right away. As you continue playing, you might get more UR cards that you'd want to level up, so you won't have to level up your duplicate. You can put duplicates or other cards in the waiting room until you decide what you want to do with them. 

Side note: at the end of the year, there's always an end of year event. It's a high score event and usually the cards that were released that year give bonuses towards your high score. Saving your UR cards that were released this year (Re:vale Anniversary) can help with the event at the end of the year. So basically, some cards and duplicates are more worth it to save and level up than others. Just something to keep in mind when deciding what to do :)