Duplicate symbol from single library in iOS4.1 SDK - xcode

Question

I am developing an iPhone application. I am unfamiliar with Xcode, so please bear with me. I have the iOS 4.1 Device SDK. When I select "Simulator" in the "Active ..." drop-down box, my application compiles without errors and runs in the iPhone simulator.
When I select "Device" in the drop-down box, however, I get the following linker error regarding a duplicate symbol:
Ld build/PineCone.build/Debug-iphoneos/PineCone.build/Objects-normal/armv6/PineCone normal armv6
cd /Users/isaacsutherland/fydp/PineCone/PineCone
setenv IPHONEOS_DEPLOYMENT_TARGET 4.1
setenv PATH "/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin:/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/gcc-4.2 -arch armv6 -isysroot /Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS4.1.sdk -L/Users/isaacsutherland/fydp/PineCone/PineCone/build/Debug-iphoneos -L/Users/isaacsutherland/fydp/PineCone/PineCone/../3rd/libGHUnitIPhone -F/Users/isaacsutherland/fydp/PineCone/PineCone/build/Debug-iphoneos -filelist /Users/isaacsutherland/fydp/PineCone/PineCone/build/PineCone.build/Debug-iphoneos/PineCone.build/Objects-normal/armv6/PineCone.LinkFileList -dead_strip -all_load -ObjC -miphoneos-version-min=4.1 -framework Foundation -framework UIKit -framework CoreGraphics /Users/isaacsutherland/fydp/PineCone/3rd/three20/Build/Products/Debug-iphoneos/libThree20.a /Users/isaacsutherland/fydp/PineCone/3rd/three20/Build/Products/Debug-iphoneos/libThree20Core.a /Users/isaacsutherland/fydp/PineCone/3rd/three20/Build/Products/Debug-iphoneos/libThree20Network.a /Users/isaacsutherland/fydp/PineCone/3rd/three20/Build/Products/Debug-iphoneos/libThree20Style.a /Users/isaacsutherland/fydp/PineCone/3rd/three20/Build/Products/Debug-iphoneos/libThree20UI.a /Users/isaacsutherland/fydp/PineCone/3rd/three20/Build/Products/Debug-iphoneos/libThree20UICommon.a /Users/isaacsutherland/fydp/PineCone/3rd/three20/Build/Products/Debug-iphoneos/libThree20UINavigator.a -framework QuartzCore -framework CFNetwork -framework MobileCoreServices -framework SystemConfiguration -lz.1.2.3 /Users/isaacsutherland/fydp/PineCone/ClientDal/build/Debug-iphoneos/libClientDal.a -lGHUnitIPhone4_0 -o /Users/isaacsutherland/fydp/PineCone/PineCone/build/PineCone.build/Debug-iphoneos/PineCone.build/Objects-normal/armv6/PineCone
ld: duplicate symbol _RedirectionLimit in /Users/isaacsutherland/fydp/PineCone/ClientDal/build/Debug-iphoneos/libClientDal.a(libASIHTTPRequest.a-armv6-master.o) and /Users/isaacsutherland/fydp/PineCone/ClientDal/build/Debug-iphoneos/libClientDal.a(libASIHTTPRequest.a-armv6-master.o)
collect2: ld returned 1 exit status
Command /Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/gcc-4.2 failed with exit code 1
The error is strange because it complains that _RedirectionLimit is found twice -- in the same file! libClientDal.a(libASIHTTPRequest.a-armv6-master.o) is the offending library. Can someone help me understand what is going on? How could this library have compiled properly in the first place? Or perhaps the linker is trying to include the same library twice?
The workaround provided in this similar question does not work for me.
If you need more information, I will gladly provide it -- as I said, I'm new to Xcode development.

Answer 1

When you have a web of projects that compile to static libraries and refer to one another, there are 2 distinct concerns you must consider:
A project's Direct Dependencies inform Xcode which projects depend on each other so it knows to recompile a project when its dependencies change.
A project's Linked Libaries actually get included in its object code.
In a nutshell, your web of direct dependencies can be as tangled as you want, but you must be careful to link each project's code into the application executable only once.
Basically, my problem was that I had 3 projects A, B, and C, and the dependencies looked like A=>B, A=>C, B=>C. I was linking libC.a into both A and B, so the linker complained about duplicate code.
The configuration stuff you need to change is on the Target Info page for each of your projects' targets.

Answer 2

This happened to me when I used the -all_load linker flag, which forces the linker to load all symbols from all libraries. The Three20 project says you should use it, because otherwise the Categories won't be loaded and you get a runtime exception. I removed that flag, and added the -force_load flag just for each library that needed it (the Three20 libraries). See also: What does the -all_load linker flag do?

Answer 3

I've several apps that need to embed a small custom library that makes use of ASI and TBXML. Some of this apps have their own version of that libraries. In order to avoid duplicate symbol issue, I duplicated the each library's target removing the .m files that causes the issue.
Hope it helps.