How does Marketo handle new leads from CRM (Salesforce) if they already exist in Marketo?

A new lead was just created in Salesforce, then synced over to Marketo. Interestingly enough, that lead was already in our Marketo database (same email, title, name, company, phone) but Marketo still created a whole new lead record for the Salesforce lead, which now has me with two lead records in Marketo (duplicate). I'm not sure if this is a one-time issue, or a larger problem, but my guess is the latter because we have tens of thousands of duplicate records in our system.

I'm a bit new to my role and system, and I have yet to tackle our duplication issue, but I was under the impression that when a new lead is created (via form, list upload, CRM sync, et al.) if Marketo found a preexisting lead with the same email address, they would just update the pre-existing record (or link it to the new CRM record) instead of creating a whole new lead. Before I reach out to support, I wanted to ping everyone here to see if they had any insights or thoughts. Could it be a configuration issue? Any settings I should look into? Things to know about how the CRM sync or dedupe process works?

And while we're on the topic, is there any easy way to mass merge duplicates that may have been caused by this method?

Thanks in advance!