Import subscriber with multiple values for custom field


I have an issue I'd love any help with overcoming.

We have a custom field set up with multiple options:
Custom Field
- custom field option 1
- custom field option 2
- custom field option 3

We have a spreadsheet ready to import with contacts who have more than 1 option for Custom Field, i.e. Contact name, Email, Custom Field - custom field option 1, custom field option 2

However when importing it won't import both options into the Custom Field in the contact.

Any help greatly appreciated!

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