AttributeValueMustBeUnique

A sync export failed because the mail, proxyAddresses, signInName or userPrincipalName value already belongs to another object.

Why it happens

Entra ID requires unique values for mail, proxyAddresses, signInName and userPrincipalName. Microsoft's example is an admin adding smtp:bob@contoso.com to one user when another user already has it.

How to fix it

1. 1

   Step 1

   Find the two objects that share the value.

2. 2

   Step 2

   Remove the duplicate from the wrong object in its source directory.

3. 3

   Step 3

   Let sync run and confirm the error clears.

Get-ADUser -Filter "proxyAddresses -eq 'smtp:bob@contoso.com'" -Properties proxyAddresses

Source

Full guide: How to move from on-premises Active Directory to Microsoft Entra ID →