IdFix -for DirSync - duplicate mailnickname attributes, should I be concerned?

I'm looking to go Exchange 2010 Hybrid, and right now I'm working to make sure my Active Directory is "cleaned properly" prior to moving forward with setting up a DirSync server.
I ran the tool recommended, IdFix, and it is reporting a number of duplicate "mailnickname" attributes, as I have two domains in a single forest.
Does that have to be corrected?

My understanding was as long as all UserPrincipalName's match their e-mail addresses (which would make them unique in our environment), then that would suffice and no other duplicate attributes would really be an issue.

Screenshot of results:
dirsyncresults
I exported to Excel and confirmed that it's finding the duplicate of this attribute between one domain and a different domain, both of which are in the same, single forest, however.
garryshapeAsked:
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Vasil Michev (MVP)Commented:
"Mail" attribute is also synced to O365, and needs to be unique, otherwise you will get errors after you run dirsync.
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garryshapeAuthor Commented:
I guess I'm not sure why the mailnickname is not being derived from the UPN or primary smtp address.    
So I have a user JDoe with mailnickname of "JDoe", whereas their upn is "JDoe2@domain.com" and their primary e-mail is JDoe2@domain.com" as well, so I don't know why mailnickname is staying as "JDoe".
Is it fine to just change the mailnickname to be JDoe2? That shouldn't cause issue with authentication or mail flow right?
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Vasil Michev (MVP)Commented:
Changing it shouldnt affect anything, as long as the ProxyAddresses attribute is populated as well. If proxyaddersses is empty, the mail attribute will be used to set the primary SMTP in O365 instead.
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