WordPress doesn't allow multiple users to share the same e-mail address, but there are certain plugins that change that behavior. This is an example workaround using the Allow Multiple Accounts plugin.
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Install and activate this plugin: https://wordpress.org/plugins/allow-multiple-accounts/
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Use a custom PHP function in the import to generate a fake e-mail for the imported user(s): [fake_email()]. Screenshot: https://d.pr/FREE/Yr0MOL.
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Store the real e-mail in a Custom Field named '_email_temp'. Screenshot: https://d.pr/ERw4F9
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Use our API to change the users e-mail to the real e-mail after the user is saved.