[R] How to selectively sum rows [Beginner question]

[asindc]

Hi, I am new to R so I would appreciate any help. I have some data that has
passenger flight data between city pairs. The way I got the data, there are
multiple rows of data for each city pair; the number of passengers needs to
be summed to get a TOTAL annual passenger count for each city pair. 

So my question is: how do I create a new table (or data frame) that
selectively sums

My initial thought would be to iterate through each row with the following
logic:

1. If the ORIGIN_WAC and DEST_WAC pair are not in the new table, then add
them to the table
2. If the ORIGIN_WAC and DEST_WAC pair already exist, then sum the
passengers (and do not add a new row)

Is this logical? If so, I think I just need some help on syntax (or do I use
a script?). Thanks.

The first few rows of data look like this:



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