I don't have animation for Mission Control anymore. It seems that it has been like that for few days. I have updated Duet a few days ago, maybe that's related...

What happened? Is it possible that someone disable it? Or, is it a bug or the sign that the hardware is damaged?

OS: 10.11.5 Hardware: MacBook Pro (2,3 GHz Intel Core i7, 16 Go, Intel HD Graphics 4000 1536 Mo)

Edit

I've tried to do in the terminal

defaults write com.apple.dock expose-animation-duration -float 0.25

then press enter

killall Dock

then press enter

It works. But when I restart the computer, it does not work anymore.

I've tried installing Duet (third party software which I suspected to be responsible). It did not change anything.

Edit 2

I've tried uninstalling Duet (third party software which I suspected to be responsible). It did not change anything.

Try resetting your dock back to it's original configuration. This will involve deleting the com.apple.dock.plist file so that when the Dock is reloaded it will create a new dock as if this is the first time you are logging in. This means you will have to redo all your customizations

Go to ~/Library/Preferences and execute the command:

mv com.apple.dock.plist ~/.Trash; killall Dock

That will bring your Dock back to the state when you first created your account so you will have to re-customize. I opted to move the files to trash, so that if you want to go back, you can restore from Trash and everything will be the way it was before you started.

However, if you just want to delete the file permanently, you could also issue the command (there's no going back with this one):

rm com.apple.dock.plist; killall Dock

Reboot, and all should be working again.

I too, have Duet installed and lost Mission Control animations on last update.

I believe it's some lazy "bug fixing" on Duet's team: Duet was running and I got logged out of my Mac

defaults delete com.apple.dock expose-animation-duration

then run Duet and then

defaults read com.apple.dock expose-animation-duration

It's again gets set to zero.

So my solution so far is deleting expose-animation-duration setting and removing Duet from autostart for a time being.