Question: How do I sum values only if a unique value exists in another column?

Answer:

Formula in D13:

=SUM(IF(COUNTIF(List1, List1)=1, List2, 0)) + CTRL + SHIFT + ENTER

The ranges don´t have to be adjacent.

Named ranges
List1 (A2:A11)
List2 (B2:B11)
What is named ranges?

How to customize the formula to your excel sheet
To copy this formula in to your excel spreadsheet you don´t need to edit the formula.  Only change the named ranges.

Download excel sample file for this tutorial.
sum-only-if-unique-value-in-another-column.xls
(Excel 97-2003 Workbook *.xls)

Functions in this article:

COUNTIF(range,criteria)
Counts the number of cells within a range that meet the given condition

IF(logical_test;[value_if:true];[value_if_false])
Checks whether a condition is met, and returns one value if TRUE, and another value if FALSE

SUM(number1,[number2],)
Adds all the numbers in a range of cells

Related posts:

1. Count unique values in two lists combined in excel
2. Count matching cell values in two columns in excel