NM: Validation Error: Account has a homeMDB value set, may already be Mailbox enabled (4311914)

Issue
An error is reported for an account when using the Run Validation Check process in Notes Migrator indicating that the account has a homeMDB value set (Mailbox Database).
This is a verification of the account in Exchange or Office 365 to determine if the account exists for provisioning a mailbox, also if the account may already be provisioned and have a mailbox configured.

Resolution
This error indicates that the target account in Exchange should be checked to verify if it already has a mailbox enabled.
The Notes Migrator processing expects there to be a Mail Enabled User object in Exchange for an account to be migrated. This object would then have a mailbox enabled during the Provisioning process (after the Validation) to allow for the migration to be completed.
In some cases, the provisioning may have already been completed for an account in Exchange directly or on a previous migration for the account.
In this case the error can be ignored as the account would be expected to have a mailbox and is therefore ready for migration.
Note: if an account does have a mailbox then the provisioning step to create a mailbox can be skipped but the Set Full Access should be completed or verified in the Exchange Admin Portal on the account's mailbox properties under the Mailbox Delegation - Full Access section to ensure that the Outlook migration account or Full Access group is applied.

Additionally, the matching for the account should be verified to ensure that the source Domino match data is not duplicated and resulting in an incorrect account being found in Exchange.
For Exchange on premise migrations the matching is completed through an Active Directory (AD) search using the Domino data field specified on the Notes Migrator Settings - User Provisioning tab for the "Match Active Directory account to" value.
For Office 365 migrations the matching is completed through PowerShell directly against Office 365 and will use the Domino Internet Address field.
To verify an account the Domino data field used should be checked for its value and that used to search AD or Office 365 to determine that the account with the mailbox is the expected target mailbox for the migration.

For Example:
The Notes ShortName is used for the matching to Exchange to locate accounts for provisioning.
The validation is completed, and the account is flagged with the "Account has a homeMDB value set, may already be Mailbox enabled".
Open the account document in Notes Migrator and check the ShortName field value.
To validate in the Exchange Admin Portal got to the Recipients - Mailboxes section and perform a search for the Notes ShortName value.
Verify if an account with a mailbox is located and if so that the account is the expected match for the Notes account (eg correct name and email address).
Then perform the same search in the other recipient categories such as contacts to ensure that a second account is not found.
Additionally, a search (Find) can be performed in AD to verify that the expected account is located.
 
If the checks are correct and the account does have a mailbox then the migration can proceed.

If the checks are not correct, then the migration should not be completed, and the account should be verified to ensure that there are not any duplicate values in the Domino Directory on multiple accounts and then the same for AD or Office 365.
This can occur if the Domino Directory has two accounts with the same ShortName in this example (or Internet Address if that is used).

Note: the migration system will not be able to rectify duplicate accounts, and this would need to be manually addressed in either the Domino Directory or the AD / Office 365 system prior to migration.