Fun Fact

[VeryMoldyMnM]

Fun fact in factorio if you hold A then hold D you will start moving right however if you hold D then hold A you will still continue to go right

[kaki_gamet]

Not a surprise, if you drive a train and reach a T junction where one can go right or left only, by holding W (forward) alone train will automatically turn right. Sort of it takes priority.

[Hurkyl]

A lot of games have this sort of priority on input. It's easier to implement, since you can just write something like "if up, do this, if down, do that, if right do something else, if left do another thing".

Some old console games, incidentally, get really confused if you can press left and right at the same time.

[Khan Boyzitbig of Mercia]

Input devices themselves often have a priority override for similar inputs.

[Nilfsen]

If someone thinks it is ineresting then same is with up/down.
Up takes priority over down.
Except if you are already running left, running left-down have priority over left-up.
Running right doesn't change up/down priority.
Running up or down doesn't change left/right priority.

[luponix]

its a design difference. some games implement them to cancel out, some give a preference to one direction