Hi,
The same what exactly ?
In System Preferences > iCloud there are not setting for iMessages, FaceTime, iTunes, The App Store, Game Center, iBooks to name some of the services that you might need an Apple ID for.
That said the iMessages ID mus also be iCloud valid even if it is not using as the iCloud ID.
iMessages account on the Mac logs into the iMessages Servers and as part of that check the ID against the Apple ID servers.
However the Mac also uses an Auth Token to "prove" that the call is from your Mac (it is based on the Serial Number)
All devices that use iMessages use the Serial Number to make a Device Specific Login.
i.e. it is impossible to spoof the Mac or iPhone or other devices.
There is not check though when adding other devices.
Someone with access to your Apple ID and password could add another device.
It can start sending Messages even before your accept the pop ups on other devices and Denying the pop only adds the device/ID used as an Alternative but in an inactive form.
Potentially there is some risk to your iCloud account if you use the Same ID in iCloud.
On the Apple ID site you can see the devices listed that people can contact you On.
The list does not point out which are iMessages, which are FaceTime or the Game Center.
They are not listed for Mail (iCloud), Photos (iCloud)
However these also cannot be removed.
At present nothing in this thread has suggested that an iCloud account (if the ID is used in both places) has been compromised other than somehow iMessages from non registered devices are getting into the system.
8:37 pm Sunday; October 9, 2016
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 iMac 2.5Ghz i5 2011 (El Capitan)
 G4/1GhzDual MDD (Leopard 10.5.8)
 MacBookPro 2Gb (Snow Leopard 10.6.8)
 Mac OS X (10.6.8),
 iPhone and an iPad (2)