Problem with dublicates and SUM Sql Query

I have 2 tables:

Customer table:
customer(nvarchar(255))

Certificates
customer(nvarchar(255))
cert(int)


Customer
570715000000038266
570715000000038266
570715000000038266
570715000000038266
570715000000038266
570715000000038266
570715000000038266
570715000000038266
570715000000038266
570715000000038266
570715000000038266
570715000000038266
570715000000038266
570715000000038266
570715000000038266

Certificate
    Customer                     cert
570715000000038266      1383
570715000000038266      1656
570715000000038266      1944
570715000000038266      -1383
570715000000038266      -1350
570715000000038266      1350
570715000000038266      -1944
570715000000038266      850
570715000000038266      -1532
570715000000038266      1532
570715000000038266      774
570715000000038266      -1656
570715000000038266      -774


When I execute this query:

SELECT     [Customer].[Customer], SUM(Certificate.Cert)
FROM         [Customer]
inner JOIN Certificate ON [Customer].[Customer] = Certificate.[Customer]
WHERE [Customer].[Customer] = '570715000000038266'
GROUP BY [Customer].[Customer]

I get this result:
 Customer                           Cert
570715000000038266    12750

It multiplies the customer records with all the certificates lines.
15 customers records X all the matching cert records = 12750

The result i am looking for is:

 Customer                           Cert
570715000000038266    850    (12750/15=850)

There are more than one customer in the customer table with multiple lines…


Hope you can help!?