I've imported a single opt in list to Klaviyo - can I send them an email to ask for double opt in?

@BoxSolution - If the users already Single Opted In to a List in Klaviyo (or from another email provider or system), and you import it, as you’re aware it doesn’t send the Double Opt-In email. There’s two approaches to this: 

Forcing a Double Opt-In Email Confirmation

If you absolutely want them to get the “Double Opt In” confirmation email,, the only way I can think of to do this is to send a Campaign email and link them to their Subscribe & Preferences Pages → Subscribe. 
Note, this means they will have to subscribe (again) and then get another email to get the Double Opt In confirmation email.

Here’s how to get the link to your Subscribe page for a List.

1. First, create a new List or reference a different List then the List your imported List is currently in.  Make sure Double Opt In is enabled for that List.
2. Go to the List, then click on “Subscribe & Preference Pages”
3. Navigate to the “Subscribe” page. 
4. Copy the URL for your Subscribe in the middle-right corner of the page:

For example, for users to subscribe to my Newsletter, you can navigate to this link:

https://manage.kmail-lists.com/subscriptions/subscribe?a=LMcvPy&g=JyrQvp

The reason you want to reference the Subscribe page of a different List is because if a user “Subscribes” to a List they are already Opted In for (from your import) it won’t trigger a Double Opt In email because they are technically already opted in. 

If you imported them into the List you want them to be in, you’ll have to remove them from the List first, import them to a different (temporary) List so you can use it to send them your Campaign email.  

Send Campaign as a Faux “Double Opt In” Method

Alternatively, you can just send them a Campaign for them to “Click a Button” to confirm the Double Opt-in.  The link can go anywhere, maybe a custom designed “Thank You Page” on your store or their “Manage Preferences” page in Klaviyo. 

You can then create a Segment of all recipients who clicked on the button, and use that Segment (or rule) to know who has “Double Opt In” - this isn’t a true “Double Opt in” email, but it serves a similar purpose to know who actually “Confirmed” the process. The nice thing about this approach is they won’t have to get yet another email. 

Hope this helps!