One For All has given Deku several extra Quirks from previous users. These abilities allow him to do a variety of things, such as create smokescreens and store kinetic energy. However, upon closer inspection, a couple of these Quirks may overlap in terms of the type of power they grant the user.
Float and Black Whip are the most obvious of these potentially redundant Quirks. At first glance, the two appear to be completely different, but given how they're used throughout My Hero Academia, one could argue that at least one of them is unnecessary. It's enough to make you wonder if Deku should have gotten both of them, or if one of them could have been replaced with a different Quirk entirely.
Where Can You Use Float and Black Whip?
Float is a pretty straightforward Quirk. It allows the user to levitate off the ground. The added strength of One For All also allows the user to propel themselves through the air at high speeds. It works a lot like Superman's ability to fly.
Black Whip allows the user to project dark tendrils from their body for a variety of purposes. They can be used from a distance to grab and bind objects. They can also be used to grab and swing from tall structures such as buildings. In this regard, they are similar to Spiderman's webs.
Deku's ability to both fly and web-swing is where the redundancy takes place. Each of these abilities can be used for getting the user where they need to be faster than they could on foot. If it's a matter of improving mobility, only one of these Quirks should be necessary per person. Deku could have been given Float or Black Whip, but he didn't need both.
However, there are a few occasions when having both Quirks can be advantageous. For example, while using Black Whip to swing around is useful in areas with tall, close-together structures, Float allows Deku to move in open areas and along the water; this application of the Quirk was particularly important in Chapter 349, "Battle Flame." In this sense, both Quirks are important for Deku's mobility.
Having both Black Whip and Float is also important for battling Shigaraki. Since this villain can disintegrate the ground and everything on it, Deku needs Float to stay off the ground and Black Whip to keep everyone else off of it. This is technically circumstantial, but since Shigaraki is the primary antagonist of the series, it's justified.
As redundant as having both flight and slinging abilities sounds, Deku has managed to make it work. The way the MHA narrative has played out has given him plenty of instances where both Float and Black Whip are good to have. With all that said, if there were ever a reason why he needed to pick one or the other, he'd probably be better off with Black Whip for how well it works in conjunction with his other Quirks.