If you don't have a decent selection of girls to use, namely 6* and/or 5* units that can bloomed to 80, I can see an argument for keeping and making use of a duplicate girl.

However; at some point you will have a good selection and will find that the duplicate is no longer needed and you'll be thinking of using it to upgrade the other and if you do that you will effectively have wasted all the manyus, upgrade materials, and gold in leveling that duplicate.

So if you decide to use a duplicate in your team, don't be in a hurry to pour upgrades into it; regard it as a placeholder.