原因：在開發過程中，遇到過上傳檔案資料上傳兩次的問題，發現是在OkHttp里加入了預設的HttpLoggingInterceptor引起的。在下面程式碼呼叫了wirteTo方法，導致會再次執行UploadFileRequestBody.writeto方法重複寫入資料。

/*
 * Copyright (C) 2015 Square, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3.logging;
import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.charset.UnsupportedCharsetException;
import java.util.concurrent.TimeUnit;
import okhttp3.Connection;
 
import okhttp3.Headers;
import okhttp3.Interceptor;
import okhttp3.MediaType;
import okhttp3.OkHttpClient;
import okhttp3.Protocol;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;
import okhttp3.ResponseBody;
import okhttp3.internal.Platform;
import okhttp3.internal.http.HttpEngine;
import okio.Buffer;
import okio.BufferedSource;
/**
 * An OkHttp interceptor which logs request and response information. Can be applied as an
 * {@linkplain OkHttpClient#interceptors() application interceptor} or as a {@linkplain
* OkHttpClient#networkInterceptors() network interceptor}. <p> The format of the logs created by
 * this class should not be considered stable and may change slightly between releases. If you need
 * a stable logging format, use your own interceptor.
 */
public final class HttpLoggingInterceptor implements Interceptor {
  private static final Charset UTF8 = Charset.forName("UTF-8");
  public enum Level {
    /** No logs. */
NONE,
/**
     * Logs request and response lines.
     *
     * <p>Example:
     * <pre>{@code
* --> POST /greeting http/1.1 (3-byte body)
     *
     * <-- 200 OK (22ms, 6-byte body)
     * }</pre>
*/
BASIC,
/**
     * Logs request and response lines and their respective headers.
     *
     * <p>Example:
     * <pre>{@code
* --> POST /greeting http/1.1
     * Host: example.com
     * Content-Type: plain/text
     * Content-Length: 3
     * --> END POST
     *
     * <-- 200 OK (22ms)
     * Content-Type: plain/text
     * Content-Length: 6
     * <-- END HTTP
     * }</pre>
*/
HEADERS,
/**
     * Logs request and response lines and their respective headers and bodies (if present).
     *
     * <p>Example:
     * <pre>{@code
* --> POST /greeting http/1.1
     * Host: example.com
     * Content-Type: plain/text
     * Content-Length: 3
     *
     * Hi?
     * --> END GET
     *
     * <-- 200 OK (22ms)
     * Content-Type: plain/text
     * Content-Length: 6
     *
     * Hello!
     * <-- END HTTP
     * }</pre>
*/
BODY
}

  public interface Logger {
    void log(String message);
/** A {@link Logger} defaults output appropriate for the current platform. */
Logger DEFAULT = new Logger() {
      @Override public void log(String message) {
        Platform.get().log(message);
}
    };
}

  public HttpLoggingInterceptor() {
    this(Logger.DEFAULT);
}

  public HttpLoggingInterceptor(Logger logger) {
    this.logger = logger;
}

  private final Logger logger;
  private volatile Level level = Level.NONE;
/** Change the level at which this interceptor logs. */
public HttpLoggingInterceptor setLevel(Level level) {
    if (level == null) throw new NullPointerException("level == null. Use Level.NONE instead.");
    this.level = level;
    return this;
}

  public Level getLevel() {
    return level;
}

  @Override public Response intercept(Chain chain) throws IOException {
    Level level = this.level;
Request request = chain.request();
    if (level == Level.NONE) {
      return chain.proceed(request);
}

    boolean logBody = level == Level.BODY;
    boolean logHeaders = logBody || level == Level.HEADERS;
RequestBody requestBody = request.body();
    boolean hasRequestBody = requestBody != null;
Connection connection = chain.connection();
Protocol protocol = connection != null ? connection.protocol() : Protocol.HTTP_1_1;
String requestStartMessage = "--> " + request.method() + ' ' + request.url() + ' ' + protocol;
    if (!logHeaders && hasRequestBody) {
      requestStartMessage += " (" + requestBody.contentLength() + "-byte body)";
}
    logger.log(requestStartMessage);
    if (logHeaders) {
      if (hasRequestBody) {
        // Request body headers are only present when installed as a network interceptor. Force
        // them to be included (when available) so there values are known.
if (requestBody.contentType() != null) {
          logger.log("Content-Type: " + requestBody.contentType());
}
        if (requestBody.contentLength() != -1) {
          logger.log("Content-Length: " + requestBody.contentLength());
}
      }

      Headers headers = request.headers();
      for (int i = 0, count = headers.size(); i < count; i++) {
        String name = headers.name(i);
// Skip headers from the request body as they are explicitly logged above.
if (!"Content-Type".equalsIgnoreCase(name) && !"Content-Length".equalsIgnoreCase(name)) {
          logger.log(name + ": " + headers.value(i));
}
      }

      if (!logBody || !hasRequestBody) {
        logger.log("--> END " + request.method());
} else if (bodyEncoded(request.headers())) {
        logger.log("--> END " + request.method() + " (encoded body omitted)");
} else {
        Buffer buffer = new Buffer();
requestBody.writeTo(buffer);//這個方法導致的資料再一次寫入
Charset charset = UTF8;
MediaType contentType = requestBody.contentType();
        if (contentType != null) {
          charset = contentType.charset(UTF8);
}

        logger.log("");
logger.log(buffer.readString(charset));
logger.log("--> END " + request.method()
            + " (" + requestBody.contentLength() + "-byte body)");
}
    }

    long startNs = System.nanoTime();
Response response = chain.proceed(request);
    long tookMs = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startNs);
ResponseBody responseBody = response.body();
    long contentLength = responseBody.contentLength();
String bodySize = contentLength != -1 ? contentLength + "-byte" : "unknown-length";
logger.log("<-- " + response.code() + ' ' + response.message() + ' '
+ response.request().url() + " (" + tookMs + "ms" + (!logHeaders ? ", "
+ bodySize + " body" : "") + ')');
    if (logHeaders) {
      Headers headers = response.headers();
      for (int i = 0, count = headers.size(); i < count; i++) {
        logger.log(headers.name(i) + ": " + headers.value(i));
}

      if (!logBody || !HttpEngine.hasBody(response)) {
        logger.log("<-- END HTTP");
} else if (bodyEncoded(response.headers())) {
        logger.log("<-- END HTTP (encoded body omitted)");
} else {
        BufferedSource source = responseBody.source();
source.request(Long.MAX_VALUE); // Buffer the entire body.
Buffer buffer = source.buffer();
Charset charset = UTF8;
MediaType contentType = responseBody.contentType();
        if (contentType != null) {
          try {
            charset = contentType.charset(UTF8);
} catch (UnsupportedCharsetException e) {
            logger.log("");
logger.log("Couldn't decode the response body; charset is likely malformed.");
logger.log("<-- END HTTP");
            return response;
}
        }

        if (contentLength != 0) {
          logger.log("");
logger.log(buffer.clone().readString(charset));
}

        logger.log("<-- END HTTP (" + buffer.size() + "-byte body)");
}
    }

    return response;
}

  private boolean bodyEncoded(Headers headers) {
    String contentEncoding = headers.get("Content-Encoding");
    return contentEncoding != null && !contentEncoding.equalsIgnoreCase("identity");
}
}

解決方法：用一個boolean值來阻止資料的第一次寫入，用於第二次實際寫入的資料判斷 顯示progressListener

import java.io.File;
import java.io.IOException;
import okhttp3.MediaType;
import okhttp3.RequestBody;
import okio.Buffer;
import okio.BufferedSink;
import okio.ForwardingSink;
import okio.Okio;
import okio.Sink;
import rx.Observable;
import rx.android.schedulers.AndroidSchedulers;
import rx.functions.Action1;
/**
 * 自定義回撥載入速度類RequestBody
 * 
 */
public class ProgressRequestBody extends RequestBody {
    //實際的待包裝請求體
private final RequestBody requestBody;
//進度回撥介面
private final UploadProgressListener progressListener;
//包裝完成的BufferedSink
private BufferedSink bufferedSink;
//每個RequestBody對應一個tag，存放在map中，保證計算的時候不會出現重複
private String tag;
//用於第二次實際寫入的資料判斷 顯示progressListener
private boolean mIsSecond;
/**
     * 建構函式，賦值
     *
     * @param file
* @param progressListener 回撥介面
     * @param tag 標記
     */
public ProgressRequestBody(File file, UploadProgressListener progressListener, String tag) {
        this.requestBody = RequestBody.create(MediaType.parse("multipart/form-data"), file);
        this.progressListener = progressListener;
        this.tag = tag;
}

    public ProgressRequestBody(RequestBody requestBody, UploadProgressListener progressListener,
String tag) {        this.requestBody = requestBody;
        this.progressListener = progressListener;
        this.tag = tag;
}

    /**
     * 重寫呼叫實際的響應體的contentType
     *
     * @return MediaType
     */
@Override
public MediaType contentType() {
        return requestBody.contentType();
}

    /**
     * 重寫呼叫實際的響應體的contentLength
     *
     * @return contentLength
     *
     * @throws IOException 異常
     */
@Override
public long contentLength() throws IOException {
        return requestBody.contentLength();
}

    /**
     * 重寫進行寫入
     *
     * @param sink BufferedSink
     *
     * @throws IOException 異常
     */
@Override
public void writeTo(BufferedSink sink) throws IOException {
        if (bufferedSink == null || tag != null) {
            bufferedSink = Okio.buffer(sink(sink));
}
        requestBody.writeTo(bufferedSink);
//必須呼叫flush，否則最後一部分資料可能不會被寫入
bufferedSink.flush();
mIsSecond = true;
}

    /**
     * 寫入，回撥進度介面
     *
     * @param sink Sink
     *
     * @return Sink
     */
private Sink sink(Sink sink) {
        return new ForwardingSink(sink) {
            //當前寫入位元組數
long writtenBytesCount = 0L;
//總位元組長度，避免多次呼叫contentLength()方法
long totalBytesCount = 0L;
@Override
public void write(Buffer source, final long byteCount) throws IOException {
                super.write(source, byteCount);
//獲得contentLength的值，後續不再呼叫
if (totalBytesCount == 0) {
                    totalBytesCount = contentLength();
}
                //增加當前寫入的位元組數
writtenBytesCount += byteCount;
                if (mIsSecond) {
                    Observable.just(writtenBytesCount).observeOn(AndroidSchedulers.mainThread())
                              .subscribe(new Action1<Long>() {
                                  @Override
public void call(Long aLong) {
                                      progressListener.onProgress(writtenBytesCount,
totalBytesCount);
}
                              });
}
            }
        };
}
}

/**
 * 上傳進度回撥類
 * 
 */
public interface UploadProgressListener {
    /**
     * 上傳進度
     * @param currentBytesCount
* @param totalBytesCount
*/
void onProgress(long currentBytesCount, long totalBytesCount);
}