Construct a query string with duplicate keys

[RobertGummesson]

I need to make a call to an API which uses duplicate parameter keys like this:

/api/feed/index?instrumentId=1&instrumentId=2&instrumentId=3

Normally, I'd use requestParameters with URLEncoding.queryString

    case requestParameters(parameters: [String: Any], encoding: ParameterEncoding)

...but because it's using a dictionary, I can't do:

[
    "instrumentId": 1,
    "instrumentId": 2,
    "instrumentId": 3,
]

This will result in:
Dictionary literal of type '[String : Any]' has duplicate entries for string literal key 'instrumentId'

How do I work around this?