can't find duplicate records

Our Marketo instance is new and I'm not seeing any syncing issues between SFDC and Marketo. However, I got duplicate error for several new leads when trying to upload leads. They all stated "{DUPLICATES_DETECTED: You're creating a duplicate record. We recommend you use an existing record instead.}"

The problem is when I searched the database, I don't see any duplicates. I tried email, name, and company name search. No duplicates for those receiving duplicate errors at all. Any insights? What am I missing? Thanks!