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Opps must have a unique name (create a workflow to set a naming convention). 

New Salesforce Org

Remove

Custom Fields

I don't know why, but a new org I recently created had custom fields in the Opportunity. This is confusing, so remove them, or at least remove all permissions from them. 

Page Layouts

Again, I don't know why a new org had 4 page layouts - just delete them all except the Opportunity Layout and start from scratch. 

Fields

Set permissions on the following fields that are not visible by default:

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