Adding ProcessId to should make the key unique

transform(_vfp.ProcessId)+SYS(2015) 

>I am creating a unique key with substr(SYS(2015),2, 9) as the value. Would it require that 2 users on 2 machines create the key at the same millisecond to get a duplicate, or even at the precise same moment would different machines render different values?
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>Thanks.