New Default Sync Feature Coming

Duplicate Attribute Resiliency is a feature in Azure Active Directory that will eliminate friction caused by
UserPrincipalName
and ProxyAddress
conflicts when running one of Microsoft’s synchronization tools.

Duplicate Attribute Resiliency will be the new default behavior across all Azure Active Directory tenants. It will be on by default for all tenants that enabled synchronization for the first time on August 22nd, 2016 or later.

Tenants that enabled sync prior to this date will have the feature enabled in batches. This rollout will begin in September 2016, and an email notification will be sent to each tenant’s technical notification contact with the specific date when the feature will be enabled.+

Once Duplicate Attribute Resiliency has been turned on it cannot be disabled.+

To check if the feature is enabled for your tenant, you can do so by downloading the latest version of the Azure Active Directory PowerShell module and running:+

Get-MsolDirSyncFeatures -Feature DuplicateUPNResiliency+

Get-MsolDirSyncFeatures -Feature DuplicateProxyAddressResiliency+

If you would like to proactively enable the feature before it is turned on for your tenant, you can do so by downloading the latest version of the Azure Active Directory PowerShell module and running:2

Set-MsolDirSyncFeature -Feature DuplicateUPNResiliency -Enable $true+

Set-MsolDirSyncFeature -Feature DuplicateProxyAddressResiliency -Enable $true

See full article Here: Identity synchronization and duplicate attribute resiliency