Reuse Async Task Swift

Question

I'm trying to reuse a method to refresh a table view from a JSON file on internet. When I'm trying to call the same method more than once I got this error :

EXEC_BAD_INSTRUCTION (code=EXC_i386_INVOP,subcode=0x0)

Can some one tell me how to do it right ?

Thanks you.

func topRefresh(sender:AnyObject){
    self.notificationsDisplayed.removeAll()

    var list=Array<Notification>()

    //request json
    let url = NSURL(string: jsonLink)!
    let session = NSURLSession.sharedSession()

    let request = NSMutableURLRequest(URL: url)
    request.HTTPMethod="POST"
    request.cachePolicy = NSURLRequestCachePolicy.ReloadIgnoringCacheData

    let paramString="username=Axel&password=test"
    request.HTTPBody = paramString.dataUsingEncoding(NSUTF8StringEncoding)

    let task = session.dataTaskWithRequest(request) {
        (data, response, error) -> Void in

        let httpResponse = response as! NSHTTPURLResponse

        if (httpResponse.statusCode == 200) {
            do{
                let jsonDict = try NSJSONSerialization.JSONObjectWithData(data!,options: .AllowFragments)

                list=self.parseJson(jsonDict)

            }catch {
                print("Error with Json: \(error)")
            }
        }else{
            print("HTTP Error")
        }

        //compare new data with past data

        while(list.count>0){
            let tmp=list.last
            for notification in self.notifications {
                if(tmp!.id==notification.severity){
                    list.removeLast()
                }else{
                    self.notifications.insert(tmp!, atIndex: 0)
                    list.removeLast()
                }
            }
        }

        self.notificationsDisplayed=self.notifications
        self.applyFilter()
        self.refreshControl?.endRefreshing()
        print("Finished")
    }
    task.resume()
}