Fact: Alternate Keys are Awesome

Why?

• Alternate keys can be used to uniquely identify a record in Dynamics 365 in place of the primary key.
• Alternate keys are automatically indexed which will optimize record retrieval when records are queried by the alternate key.
• A maximum of 5 alternate keys are allowed per entity.
• Alternate keys must be unique when a value is provided however a value is not required.
• Alternate key definitions may be reversed if needed.

So, if you have fields that must be unique (keys to other systems or account numbers for example) you can define those columns as Alternate Keys and let CRM enforce uniqueness for you!

Read More: https://msdn.microsoft.com/en-us/library/dn932139.aspx