You use the Change Owner or Sync Person to SFDC flow step to assign a record to an owner but when entering the intended owner's name in the flow step to select it from the suggestion dropdown, the user appears twice. Later, when running records through the flow step after selecting either option, you get an error. In SFDC you see that there are two active users with the same name.
Sales Owner names should be unique in SFDC for Marketo to be able to run records through this flow properly (e.g. if there are two users named "Marketo Sales", making one "Marketo Sales1" while leaving the other unchanged will correct the issue after the updated users sync over to Marketo.)