When the Googlebot visits your website, it saves the content to their databases. Somewhere along the line (usually it will take 72 hours to crawl) your content gets compared with other contents in their database which are stored in their DB. If the contents match from them, Google may decide your content is a duplicate. Then Google decides which version of the content is the most authoritative and deserving of being shown to the searchers and only that page will index on Google and other pages may not consider.
If you have multiple versions of the domains with the same content, you must to do the canonicalization. this is the only solution to avoid duplicate content issues.
Hope you can understand my point, Do you know what is the meaning of canonical tag? if not please drop me out!