Fix the repeated columns generated by people export

[mabdellatif88]

Describe the bug
When generating people export sometimes it repeats columns

To reproduce
Steps to reproduce the behavior:

1. Navigate to the admin interface of PGP: https://geniza.princeton.edu/admin/
2. Login
3. Click into the people module
4. Search for a person who is related to more than one place (e.g. Nahary)
5. Check at least 1 person with related places
6. From the combobox at the top, choose "Export selected people to CSV" and hit Go
7. Refresh the page
8. Repeat steps 5 and 6
9. Open the CSV file that is downloaded the second time and scroll through the places columns they are repeated

Expected behavior
On refresh and redownload, places columns shouldn't repeat

Screenshots
How to generate:

Generated CSV export:

Device information
Happens on all browsers and devices

[blms]

Confirmed fixed on staging: Refreshing 3 times with the same and different selections of people with related places, no extra columns appear in the CSVs. 👍