Why this diode exist? Why not using one diode instead?
Is it a matter of forward voltage drop and current whistanding capacity?
The parallel circuit tolerates twice the forward current of one diode.
Depending on temperature and tolerances, there is a difference in the forward voltage drop.
A good guess (in my opinon) is that the other one caries only half of the current.
It is a Schottky diode, the lower forward voltage drop is minimal.
Using two Schottky diodes parallel is a questionable design.
I wanted to use this for reverse polarity protection in a 12 volts ±30W system (already include a 3 amp fuse). The low voltage drop and high current rating (they say 10 amp) initially trigger my attention.
Only 30 Watts ? then it is okay. Those two diodes are for 5A and a little more. It has a maximum reverse voltage of 60V, that should be good enough for most situations.
Who is "they" ?
A typical scenario is someone having multiple diodes parallel and with a high current, the diodes blow one by one. That is because the diode with the lowest forward voltage will carry the most current. That diode will get hot, causing the forward voltage to lower even more, causing it to draw even more current than the other diodes. And so on.
[UPDATE]
I found "they": https://www.aliexpress.com/item/4001049749961.html
Well, "they" are full of :poop:
A guy named aliexp....ss or something 
Oh I see you find them too!
Ok I get it. So I should better rely on one SS56 diode. Having a second will not hurt but will not help very much.
A second one will help, but you will never know for how much 
Two of these were cheaper than one of the proper size? It was a copy paste of another PCB and faster to just throw this in than design it properly? Low skill level?
Many possible reasons.
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