Sum Amount Only Where Customer Number Is Distinct

Hi

Here is my sample data:

The Amount column is a repetative number and only value is needed but the dataset is provided as above.

What I am trying to accomplish:

I want to be able to produce a total amount that the customer has but only where the Customer Number is distinct. So if a Customer Number appears more than once I want to only include one amount value and disregard the remaining ones.

With regards to the table above, the correct amount total should be 135 but every DAX measure i try to produce will only ever show a total of 300.

Here is the code I thought would be sufficient but it is not working as I expected.

Distinct.Sum = CALCULATE( SUM( Table1[Amount] ) , DISTINCT( Table1[Customer Number] ))

As always, any assistance is greatly appreciated.
Paul