As per microsoft documentation on UniqueIdentifier, This value is always a unique globally beacuse it's based on network clock and CPU clock time and on the other hand the same documentation says 

> 
> uniqueidentifier columns may contain multiple occurrences of an
>  individual uniqueidentifier value, unless the UNIQUE or PRIMARY KEY
>  constraints are also specified for the column.
> 
> 
> 

I'm not able to come to a conclusion how UniqueIdentifier (GUIDs) can be unique globally, as Network address (Mac address) can be same on two different networks, How GUIDs can be unique globally with which combinations and also why Microsoft says there should be primary or unique constraint in order to ensure we've unique UniqueIdeitifer value always.

<https://technet.microsoft.com/en-us/library/ms190215(v=sql.105).aspx>