Handle Multiple Query Parameters with the Same Key

Duplicate Query Parameter Keys

URLs can contain the same query parameter key multiple times. This is valid and commonly used to represent arrays or multi-select values.

Example

https://shop.example.com/products?color=red&color=blue&color=green&size=L

How Different Environments Handle Duplicates

JavaScript (URLSearchParams):

const params = new URLSearchParams("color=red&color=blue&color=green");
params.get("color");     // "red" (first value only)
params.getAll("color");  // ["red", "blue", "green"]

Express.js / Node.js (qs library):

// req.query.color → ["red", "blue", "green"]

PHP:

// $_GET['color'] → "green" (last value wins!)
// Use color[] to get an array:
// ?color[]=red&color[]=blue → $_GET['color'] → ["red", "blue"]

Python (urllib.parse):

from urllib.parse import parse_qs
parse_qs("color=red&color=blue")
# {'color': ['red', 'blue']}

Key Differences

Environment	?a=1&a=2 result
URLSearchParams.get()	"1" (first)
URLSearchParams.getAll()	["1", "2"]
PHP $_GET	"2" (last)
Express/qs	["1", "2"]
Python parse_qs	{"a": ["1", "2"]}
Go url.Values	["1", "2"]

Best Practices

1. Be explicit — use getAll() when you expect multiple values
2. Document conventions — if your API uses repeated keys for arrays, document it
3. Framework awareness — know whether your backend gets the first value, last value, or an array
4. Consistency — pick one convention (repeated keys vs bracket notation) and stick with it