I don't know how to do that with Airtable's automations, but I'm sure that it can be done with a custom Javascript script. I don't know Javascript, but hopefully someone will chime in below to help you write a script.

In the meantime, I have all of my Airtable consulting clients doing this exact thin by using Make's Table Aggregator. Check out the screenshot below, and you can see at the bottom that there is a "Group By" function. That's how you can merge rows that have identical values into one row in your table.

Alternatively, you may be able to restructure your database in such a way where each city only shows once in the table that you're pulling your emails from.