Formula To Add Unique Values

here's the situation and what i'm trying to do:

each row/record is as follows:

column A contains zip code
column B contains zip code population
column C contains city name
columns D contains state name

there are multiple duplicates in each column.

i am trying to calculate the population for each city and place that data in each row in column E.

in order to do so, i need to add the populations (column B) of all UNIQUE zip codes (column A) for each city (column B) / state (column D).