If you’re trying to save global settings in the Magento admin and you get the following errors this is due to duplicate customer rows. If you’re certain you don’t want duplicate rows you’re able to remove them by running the following code within your MySQL database.
You should only do this if you’re certain that you don’t need the duplicate rows. Perhaps from a bad import etc.
SET SESSION old_alter_table=1;
ALTER IGNORE TABLE customer_entity ADD UNIQUE INDEX duplicate_email(email);
This will set up a unique index on the customer_entity table to stop any duplicate rows, the use of IGNORE will delete any duplicates that are already in the system. As I’ve said, only run this if you’re certain you want to remove the duplicates. A backup of the data is strongly advised.