One request - Two responses. Which end is creating the problem (Front / Back) - swift3

Question

Problem:
When I call a request from iOS swift based app, then the server is responding two responses.
Inputs:
In my request, am sending some user values including one base64 image string. I already ensure that my app is calling request only one time.
Outputs:
When we opened the server log, it printed two set of request and response. But, difference that the first one is not having base64 image string and second one is having it. Thats why am receiving two different responses.
Questions:
What end is causing this problem - Front / back end?
Note:
I given front code below but I can’t provide back end code.
let task = urlSession.dataTask(with: urlRequest, completionHandler: {
(data, response, error) in
if error != nil
{
print("Error ==",error!.localizedDescription);
onFailure(error!.localizedDescription)
}
else
{
let httpResponse = response as! HTTPURLResponse
let statusCode = httpResponse.statusCode
// For some critical cases:
//print("Status code: ", statusCode)
//print("http Response: ", httpResponse)
// JSON serialize
do {
let jsonResponse = try JSONSerialization.jsonObject(with: data!, options: .allowFragments)
print("Server Response == ",jsonResponse)
onSuccess(statusCode, jsonResponse)
}
catch
{
onFailure("JSON Parser Error")
}
}
})`