Connecting Azure AD with on-premise active directory using Azure Ad connect , possiblity of user duplication

Question

If the current tenant containing office 365 users is synchronized with on premise active directory , there is a possibility in duplication of on premise users with office 365 users in azure ad ?

In case scenario: if there is already user xyz@jaswant .com in azure ad directory , if it synchronized ,it takes on prem active directory user ie xyz@jaswant .local and register as xyz@jaswant .onmicrosoft.com will there be a duplication in users , as there will be two users after sync xyz@jaswant .com and xyz@jaswant .onmicrosoft.com?

Answer 1

It will do a soft match:
When you install Azure AD Connect and you start synchronizing, the Azure AD sync service (in Azure AD) does a check on every new object and tries to find an existing object to match. There are three attributes used for this process: userPrincipalName, proxyAddresses, and sourceAnchor/immutableID. A match on userPrincipalName and proxyAddresses is known as a soft match. A match on sourceAnchor is known as hard match. For the proxyAddresses attribute only the value with SMTP:, that is the primary email address, is used for the evaluation.

https://learn.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-install-existing-tenant#sync-with-existing-users-in-azure-ad

P.S. You can't use a .local for the on-prem UPNs. They have to be a valid domain suffix registered in Azure.