I recently updated to Android Studio 2.3, and now when I try to run the application, the emulator does not come online. It times out after 300 seconds.
Additionally,
I was having the same problem as you, android studio said I had Emulator 25.3.1 already installed
I deleted the Emulator folder (it's inside the SDK folder)
And then reinstalled Emulator 25.3.1 Sdk manager--> SDKtools --> instal the emulator 25.3.1
This time it worked :)