Is Secure.ANDROID_ID unique for each device?

With Android O the behaviour of the ANDROID_ID will change. The ANDROID_ID will be different per app per user on the phone.

Taken from: https://android-developers.googleblog.com/2017/04/changes-to-device-identifiers-in.html

Android ID

In O, Android ID (Settings.Secure.ANDROID_ID or SSAID) has a different value for each app and each user on the device. Developers requiring a device-scoped identifier, should instead use a resettable identifier, such as Advertising ID, giving users more control. Advertising ID also provides a user-facing setting to limit ad tracking.

Additionally in Android O:

• The ANDROID_ID value won't change on package uninstall/reinstall, as long as the package name and signing key are the same. Apps can rely on this value to maintain state across reinstalls.
• If an app was installed on a device running an earlier version of Android, the Android ID remains the same when the device is updated to Android O, unless the app is uninstalled and reinstalled.
• The Android ID value only changes if the device is factory reset or if the signing key rotates between uninstall and
  reinstall events.
• This change is only required for device manufacturers shipping with Google Play services and Advertising ID. Other device manufacturers may provide an alternative resettable ID or continue to provide ANDROID ID.

Check into this thread,. However you should be careful as it's documented as "can change upon factory reset". Use at your own risk, and it can be easily changed on a rooted phone. Also it appears as if some manufacturers have had issues with their phones having duplicate numbers thread. Depending on what your trying to do, I probably wouldnt use this as a UID.