Reason to connect a bidirectional tvs diode in paralell with a unidirectional one
D3 and D4 in the original design are optional so, you decide which one you need and fit it as necessary. Both are on the PCB for your pleasure. Read the Datasheet carefully and you’ll see what I mean.

Transient Protection The HSS-MOTHERBOARDEVM comes with large
footprints for TVS diodes if they are needed for any transient
testing. D3 is an SMB footprint while D4, D5, and D6 are SMC
footprints