If using a query parameter name multiple times, only the last value is actually used.
e.g. .to("cxfrs:http://example.com/") with Exchange value Exchange.HTTP_QUERY id=1&id=2 will call to example.com with id=2
As far as i can tell the problem is caused by CxfRsProducer::getQueryParametersFromQueryString. In the method all query paramters are inserted in a map, and only the last use of a parameter name is preserved.
One solution would be to insert the values directy into the client field, skipping the Map.