[2.1] Can lists have duplicate items? Eg Labels

[sr@ctin.us]

Fwd from: https://github.com/oasis-tcs/cti-stix2/issues/169

Can lists have duplicate items? Consider labels, can labels property  
have multiple duplicate values ? [1,1,2,3,4]

See the further comments from community members in the github issue  
for further context

This issue is specifically of interest because of the many lists in  
SCOs which ~"could" support duplicates or could be argued that they do  
not support duplicates.