How do I avoid duplicates across all modules  when I don't always have the email address?

I've been importing leads from spreadsheets, many times we haven't found the contact information yet, especially because sometimes there is more than one contact to reach out to.

When importing the leads, I have all the email fields come in as " noemail@domainname.com". 

Here's where I end up with lots of duplicates:

When I find an email for a contact (for example: Sportscars.com)  I would change the lead email to: Contact@Sportscars.com . That then leaves me eligible to re-enter  noemail@Sportscars.com because there's no longer a duplicate email. However now I have two leads (or sometimes 1 potential and 1 lead) for sportscars.com, which I want to avoid.

Please help me! Thanks