Merge Duplicate Customers
Open the Customer Card of the customer that will be kept moving forward (example with Adatum Corporation).
1) Select "Merge with..." from the navigation menu:
2) On the page that opens up, select the customer that we need to merge with from the drop down:
Once selected, it can take a while for the data to populate. This is normal. There are a lot of records that need to be compared.
3) Select "Merge" from the navigation menu:
If there are any conflicts, the system will show them and ask for input on how to resolve them (delete or rename).
In this example, the result of the operation will be have only customer 10000 in the list. Customer 20000 will no longer exist after the operation has been completed.

For more details, this is a link to Microsoft's article:
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