One lead, many companies

You would have to have multiple fields (not so good) or duplicate records in your CRM.

Duplicate records are really the only way to track this.

If they are with multiple firms, I imagine they'd have different emails for each one, yes?

You can still have same email for diff records as long as they come in from the CRM.

This is something we deal with in our organization.  An individual can work for a company, and be a board member of another company, and have a second email, which is their personal email because they prefer to receive some of our emails through their personal emails.  What we do is if they work for a company and if they are a board member or consultant of another company and use the same email address that's related to their company, we mark them as purposeful duplicates. For the ones that have the personal email, we respect their request, but I do ask our sales associates to reach out to them and see if it's ok if we can send their paid subscription emails to their company email.  If they say yes, we retain the personal email in the secondary email.  This way, if they leave the company, we at least have their secondary email.  

Andrew,

As others have noted, duplicate leads are probably your best bet - most companies track opportunities at the company/account level after conversion, so separate companies will have separate opportunities, and you'll want to have different leads to convert to those accounts.

One thing to keep in mind: we've experienced sending issues with our duplicates. Due to the records having unique Marketo IDs and unique Salesforce IDs they've been receiving mulitple emails regardless of Marketo's deduping upon send. So if you're experiencing many duplicates, you may want to come up with a workflow like Michelle suggested. We're in the process of figuring out a solution to manage the data as well.

@Kristen-
It's interesting that you're experiencing that since I just posed the duplicate email send question to Marketo Support.  They assured me that if the duplicate records have the same email address, but both qualify for the campaign, they would still only receive one copy of the email.  I tested it just once and found that to be true, but I wasn't fully convinced. 

We have roughly 20,000 duplicates in our system (I have nightmares about this) - most of them created on purpose due to multiple account ties.  Unfortunately, I don't think we can rely on our sales org (600+) to keep the data under control through the process Michelle suggested.  If anyone does come up with a clever way to handle this, please continue to post!

@Jessica A lot of it has to do with the unique IDs that our proprietary data system is assigning the leads. We put in a similar support ticket and Marketo told us there was nothing they could do about it. I have nightmares too. :) We're working to ameliorate this issue, but it's been a struggle.