Microsoft Dynamics CRM data import from spreadsheet

I just wanted to share a quick "best practice" when importing data into CRM. 

I have a client with a bunch of spreadsheets of transactions they would like to have in CRM. They created an entity and started importing them, then discovered that some records failed, and some appear to be in the system more than once (duplicates). This turned into a huge mess because the transactions all look very similar and there is no combination of fields that make them unique. 

One way they could figure out what is in the system and what is not, is if they had created a 'primary key' field in their spreadsheets and mapped that to a field in CRM. At least that way you could search and see if you had duplicate primary keys (should never happen) or even set duplicate rules to prevent them from being imported in the first place. Just my $.02