That depends on what the target server accepts. There is no definitive standard for this. See also a.o. Wikipedia: Query string:
While there is no definitive standard, most web frameworks allow multiple values to be associated with a single field (e.g.
field1=value1&field1=value2&field2=value3).[4][5]
Generally, when the target server uses a strong typed programming language like Java (Servlet), then you can just send them as multiple parameters with the same name. The API usually offers a dedicated method to obtain multiple parameter values as an array.
foo=value1&foo=value2&foo=value3
String[] foo = request.getParameterValues("foo"); // [value1, value2, value3]
The request.getParameter("foo") will also work on it, but it'll return only the first value.
String foo = request.getParameter("foo"); // value1
And, when the target server uses a weak typed language like PHP or RoR, then you need to suffix the parameter name with braces [] in order to trigger the language to return an array of values instead of a single value.
foo[]=value1&foo[]=value2&foo[]=value3
$foo = $_GET["foo"]; // [value1, value2, value3]
echo is_array($foo); // true
In case you still use foo=value1&foo=value2&foo=value3, then it'll return only the first value.
$foo = $_GET["foo"]; // value1
echo is_array($foo); // false
Do note that when you send foo[]=value1&foo[]=value2&foo[]=value3 to a Java Servlet, then you can still obtain them, but you'd need to use the exact parameter name including the braces.
String[] foo = request.getParameterValues("foo[]"); // [value1, value2, value3]