Re: using Insert Ignore for a specific column only

[Phillip Ward]

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... the email entered by the administrator to identify an employee.

Identifying implies uniqueness.
No employee should be using the same email address to work for more than one company (that's just a Data Breach waiting to happen!).

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Can I index on two fields to stop dupes?

Yes, you can but just be aware that this is recording a different relationship.
An employee probably can take part in any number of campaigns, but each campaign only once, so a unique index on ( campaign, employee_email ) would cover that.

Regards, Phill W.