Why duplicated keys?

[Entropia]

I understand that having two shift keys may be useful for typists, but what is the reason keyboards have two control keys? I think I've never ever used the right control key. Do you really make combinations with the right control depending on how close the letter is? Also, I've seen many people appreciate the Windows key and even include it in 60% keyboards. I think the only time I've ever used it is for watching the cool effect of windows swaping in perspective when using Windows + tab, which is something that can be done (and faster) with alt + tab. Do you really use the Window key for anything useful that I ignore. Oh, and back to the topic: not one but two Windows keys? I don't get it. Well, at least they can be remaped to do something useful with Autohotkey. Same with scroll lock and pause, which I use for screen dim and pause/resume my music.

[Fictiouz]

I understand that having two shift keys may be useful for typists, but what is the reason keyboards have two control keys? I think I've never ever used the right control key. Do you really make combinations with the right control depending on how close the letter is? Also, I've seen many people appreciate the Windows key and even include it in 60% keyboards. I think the only time I've ever used it is for watching the cool effect of windows swaping in perspective when using Windows + tab, which is something that can be done (and faster) with alt + tab. Do you really use the Window key for anything useful that I ignore. Oh, and back to the topic: not one but two Windows keys? I don't get it. Well, at least they can be remaped to do something useful with Autohotkey. Same with scroll lock and pause, which I use for screen dim and pause/resume my music.

I think it's due to hand dominance and preference. My friend plays video games with I-J-K-L and uses right control for gaming. Maybe lefties have a preference to it. Also the way a keyboard is technically "supposed to be used" is to modify with one hand and type with the other.

[Findecanor]

In the touch-typing method, you hold down a modifier with one hand while pressing the key with the other hand.
The custom started with Shift, and I suppose that that custom just transferred to other keys.

Not all keyboards have two Control keys though. Many keyboards for terminals, Unix systems, Apple II and the very first IBM PC had the Control key to the left of A where most modern keyboards have a Caps Lock key.
Most operating systems allow the Control key to be mapped there and many modern keyboards even do it in hardware. A couple keyboards even come with duplicate keycaps for swapping the legends between left Control and Caps Lock as well.

In most European layouts, the left and right Alt keys are not the same key: the right Alt is "Alt Graph" for making special "graphical" characters. For instance, the characters { [ ] and } are on Alt Gr + 7 through 9.

Oh, and back to the topic: not one but two Windows keys?

If a key is available on a keyboard that they use, that person might start become accustomed to it... I have seen users here complain about keyboards that don't have a right Windows key. (Some have a right Fn and Menu keys instead for some reason)

[Tye]

Do you really make combinations with the right control depending on how close the letter is?

Yes, when touch typing, I do. Even when not touch typing, its useful for things like holding a pen in my left hand to take notes while using ctrl-shift-arrow cluster to select text. Alt-Tab is also less hand contorting if you use both sides (assuming "proper" typing technique, of course)

[Data]

Ctrl-key macros are usually used with the mouse, and most of them are positioned on the left half of the keyboard.  So that's why the Right Ctrl never gets used.

Example:
Mouse Highlight,
Ctrl+C,
Click,
Ctrl+V,
Ctrl+A,
Ctrl+X,
Ctrl+Z,
Ctrl+S,
Alt+F4.

Your hands basically never move.  Other macros tend to require two hands but they're a little more situational.  Ctrl+O, Win+L, etc.

Games likewise reinforce left-handed keyboarding habits.   So yeah, the Right Ctrl and (to a slightly lesser extent) Right Alt are redundant keys.

[menuhin]

Ctrl-key macros are usually used with the mouse, and most of them are positioned on the left half of the keyboard.  So that's why the Right Ctrl never gets used.

Example:
Mouse Highlight,
Ctrl+C,
Click,
Ctrl+V,
Ctrl+A,
Ctrl+X,
Ctrl+Z,
Ctrl+S,
Alt+F4.

Your hands basically never move.  Other macros tend to require two hands but they're a little more situational.  Ctrl+O, Win+L, etc.

Games likewise reinforce left-handed keyboarding habits.   So yeah, the Right Ctrl and (to a slightly lesser extent) Right Alt are redundant keys.

Some advanced Emacs users would advocate the usage of Right Ctrl.
However, Right Alt is essential for most of the European users, it's the "Alt Gr" to access special symbols. If "Alt Gr" moves to the LHS and if the user does not assign Alt-key macros to pre-assigned symbol key combos (e.g. Alt_Gr + E for € or Alt_Gr + Q for @, and these: ², ³, {, [, ], }, \, ~). Therefore, I still hesitate to design a layout for my keyboards without the Alt Gr, i.e. the Right Alt.

[opensecret]

Also, I've seen many people appreciate the Windows key and even include it in 60% keyboards. I think the only time I've ever used it is for watching the cool effect of windows swaping in perspective when using Windows + tab, which is something that can be done (and faster) with alt + tab. Do you really use the Window key for anything useful that I ignore.

In both Win8 and Win10, the Windows key toggles between the old and the new -- the desktop and the start menu.  I use it all the time for that purpose.  You can do the same thing with Ctrl-Esc or by clicking Start in Win10, but the Win key is quicker and easier.   The Win key is particularly useful in Win8, which doesn't have the start button (which is one of the reasons I was in the distinct minority that found Win8 easy to use and an improvement over Win7).  There are also a number of useful keyboard macros that involve the Windows key:  Win + R to open a run dialogue, Win + M to minimize all windows, etc.