duplicate symbol _OBJC_METACLASS_$_GTMBase64
导入框架时遇到这个问题，开始一直以为是路径之类的问题，一直没有得到解决后来网上查询资料，得到是因为引用库重复定义的原因
网上查询资料参考
http://hmifly.blog.163.com/blog/static/128583507201342752241781/
PS:以下是直接的原文，留着自己看了！
项目需要，同时引用ZBar和QQ授权登录SDK，由于二者均使用了Base64处理数据，XCode编译时报错：
duplicate symbol _base64_encode in：
...\libzbar.a(symbol.o)
...\TencentOpenAPI(base64.o)
意思就是在这两个库中都定义了_base64_encode，编译器认为你重复定义了。至于为什么编译器认为重复定义，个人认为编译器编译全局变量时会根据名字进行编译，会把相同名称的全局变量编译为相同变量，也就是多个编译成一个，而编译器认为这样可能会引起错误，就提醒用户这里有错。
解决方案：
参考了：http://blog.sina.com.cn/s/blog_4beb28f301012bl6.html
删掉了 set building->other linker flag-> -all_load
ios的Framework是共享动态库，不会被打包到app中，非系统Framework静态库都会被打包到app中，所以会产生"Duplicate Symbol"的错误。
在Build Settings->Other link flags中删除所有的-all_load与-force_load, XCode会很smart的去掉"Duplicate Symbol"。
以下是从外国友人那获取的终极解决策略，方案是修改类库：
I'm
going to assume that these are two third party libraries that have only
provided you with the .a files and not the source code. You can use
libtool, lipo and ar on the terminal to extract and recombine the files.
假设有两个三方类库仅提供给你了.a文件，没有源码，你可以通过libtool, lipo和ar在terminal中解压合并他们。
To see what architectures are in the file:
查看文件都支持了什么架构
$ lipo -info libTapjoy.a
Architectures in the fat file: libTapjoy.a are: armv6 i386
Then to extract just armv6, for example:
然后只解压armv6，例如
$ lipo -extract_family armv6 -output libTapjoy-armv6.a libTapjoy.a
$ mkdir armv6
$ cd armv6
$ ar -x ../libTapjoy-armv6.a
You can then extract the same architecture from the other library into the same directory and then recombine them like so:
你可以从另一个类库中解压同样架构的部分，然后将两者合并在一起
$ libtool -static -o ../lib-armv6.a *.o
And then finally, after you've done this with each architecture, you can combine them again with lipo:
如上所示，你可以将所有架构都按照这个流程走一遍，然后合并
$ cd ..
$ lipo -create -output lib.a lib-armv6.a lib-i386.a
This should get rid of any duplicate symbols, but will also combine the two libraries into one. If you want to keep them separate, or just delete the duplicate from one library, you can modify the process accordingly.
假设有两个三方类库仅提供给你了.a文件，没有源码，你可以通过libtool, lipo和ar在terminal中解压合并他们。
To see what architectures are in the file:
查看文件都支持了什么架构
$ lipo -info libTapjoy.a
Architectures in the fat file: libTapjoy.a are: armv6 i386
Then to extract just armv6, for example:
然后只解压armv6，例如
$ lipo -extract_family armv6 -output libTapjoy-armv6.a libTapjoy.a
$ mkdir armv6
$ cd armv6
$ ar -x ../libTapjoy-armv6.a
You can then extract the same architecture from the other library into the same directory and then recombine them like so:
你可以从另一个类库中解压同样架构的部分，然后将两者合并在一起
$ libtool -static -o ../lib-armv6.a *.o
And then finally, after you've done this with each architecture, you can combine them again with lipo:
如上所示，你可以将所有架构都按照这个流程走一遍，然后合并
$ cd ..
$ lipo -create -output lib.a lib-armv6.a lib-i386.a
This should get rid of any duplicate symbols, but will also combine the two libraries into one. If you want to keep them separate, or just delete the duplicate from one library, you can modify the process accordingly.
这个过程不仅解决掉了duplicate symbols的问题，也将两个类库合并为一个。如果你想分别保存两个类库，你可以将duplicate的部分从任意一个类库中删除，你可以相应的修改这个过程。
总结：按照上述删掉-all_load后问题得到解决！
