I get the following error:
ld: duplicate symbol _TTCreateNonRetainingArray in /Users/macuser/Dropbox/DFJ/three20/Build/Products/Debug-iphoneos/libThree20Core.a(TTGlobalCore.o) and ../three20/Build/Products/Debug-iphoneos/libThree20Core.a(TTGlobalCore.o) collect2: ld returned 1 exit status Command /Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/g++-4.2 failed with exit code 1
It seems to be referring to the same .o from an absolute and relative paths.
Ld build/DFJ.build/Debug-iphoneos/DFJ.build/Objects-normal/armv6/DFJ normal armv6 cd /Users/macuser/Dropbox/DFJ/DFJ setenv IPHONEOS_DEPLOYMENT_TARGET 3.2 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/g++-4.2 -arch armv6 -isysroot /Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS4.2.sdk -L/Users/macuser/Dropbox/DFJ/DFJ/build/Debug-iphoneos -F/Users/macuser/Dropbox/DFJ/DFJ/build/Debug-iphoneos -F/Users/macuser/Dropbox/DFJ/DFJ/../../../Downloads/gdata-objectivec-client-1.11.0/Source/build/Debug -filelist /Users/macuser/Dropbox/DFJ/DFJ/build/DFJ.build/Debug-iphoneos/DFJ.build/Objects-normal/armv6/DFJ.LinkFileList -dead_strip -force_load ../three20/Build/Products/Debug-iphoneos/libThree20Core.a -force_load ../three20/Build/Products/Debug-iphoneos/libThree20UICommon.a -force_load ../three20/Build/Products/Debug-iphoneos/libThree20Launcher.a -force_load ../three20/Build/Products/Debug-iphoneos/libThree20UINavigator.a -force_load ../three20/Build/Products/Debug-iphoneos/libThree20.a -force_load ../three20/Build/Products/Debug-iphoneos/libThree20UI.a -force_load ../three20/Build/Products/Debug-iphoneos/libThree20Network.a -force_load ../three20/Build/Products/Debug-iphoneos/libThree20Style.a -all_load -miphoneos-version-min=3.2 /Users/macuser/Dropbox/DFJ/three20/Build/Products/Debug-iphoneos/libThree20Core.a /Users/macuser/Dropbox/DFJ/three20/Build/Products/Debug-iphoneos/libThree20UICommon.a /Users/macuser/Dropbox/DFJ/three20/Build/Products/Debug-iphoneos/libThree20Launcher.a /Users/macuser/Dropbox/DFJ/three20/Build/Products/Debug-iphoneos/libThree20UINavigator.a /Users/macuser/Dropbox/DFJ/three20/Build/Products/Debug-iphoneos/libThree20.a /Users/macuser/Dropbox/DFJ/three20/Build/Products/Debug-iphoneos/libThree20UI.a /Users/macuser/Dropbox/DFJ/three20/Build/Products/Debug-iphoneos/libThree20Network.a /Users/macuser/Dropbox/DFJ/three20/Build/Products/Debug-iphoneos/libThree20Style.a -framework QuartzCore -framework Foundation -framework UIKit -framework CoreGraphics -lsqlite3.0 -framework MobileCoreServices -lxml2 -framework Security -framework SystemConfiguration -framework OpenAL -framework CoreAudio -framework AudioToolbox -o /Users/macuser/Dropbox/DFJ/DFJ/build/DFJ.build/Debug-iphoneos/DFJ.build/Objects-normal/armv6/DFJ
ld: duplicate symbol _TTCreateNonRetainingArray in /Users/macuser/Dropbox/DFJ/three20/Build/Products/Debug-iphoneos/libThree20Core.a(TTGlobalCore.o) and ../three20/Build/Products/Debug-iphoneos/libThree20Core.a(TTGlobalCore.o) collect2: ld returned 1 exit status Command /Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/g++-4.2 failed with exit code 1
See the last message of the developer of Three20 here. Basically, download the newest Three20 sources and follow closely the steps described in their README.
If you are following a Three20 tutorial that is not coming from the developer, chances are it's outdated.