Hi Feather,
What action are you hoping to take on these leads now? Do you have phone number or other contact level information that you will be using to contact them?
I'd recommend not importing the list until you have email addresses and refrain from importing those who still do not have email address.
If you're set on importing the leads without email addresses, know that since marketo automatically dedupes on email address they won't be deduped, as Josh mentioned.
You can change the smart list on your possible duplicates smart list to say 'duplicate fields, field name is, full name. Just be careful when deduping off of that list since there are many common names that may appear to be duplicates. You may also add a filter 'email address is empty' if you know all the leads you want to dedupe off of full name do not have an email address.
Thanks,
Tara