iPhone Application Linking/Compile Errors

I'm working on an iPhone application. I added a new framework to my project, and this caused a bunch of linking errors, with the following error for each framework I'm using:

ld: warning: in /Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS3.1.sdk/System/Library/Frameworks/Foundation.framework/Foundation, missing required architecture i386 in file

This caused other errors, saying things like NSString did not exist. I deleted and re-added all frameworks several times to no avail. As another attempt to fix the problem, I copied all my source files to a new directory and created a new project. When trying to compile in that project, I get the error:

ld: duplicate symbol .objc_class_name_${PROJECT_NAME}AppDelegate in /Users/nick/Documents/SVN/Programming/iPhone/${PROJECT_NAME}/newTrunk/${PROJECT_NAME}/build/${PROJECT_NAME}.build/Debug-iphonesimulator/${PROJECT_NAME}.build/Objects-normal/i386/${PROJECT_NAME}AppDelegate-B70D3802A4DFAE4D.o and /Users/nick/Documents/SVN/Programming/iPhone/${PROJECT_NAME}/newTrunk/${PROJECT_NAME}/build/${PROJECT_NAME}.build/Debug-iphonesimulator/${PROJECT_NAME}.build/Objects-normal/i38开发者_如何学运维6/${PROJECT_NAME}AppDelegate-B70D3802A4DFAE4D.o

(I've replaced parts of the above with ${PROJECT_NAME} because I'm not currently able to discuss specifics of the project.)

If anyone has any insights into either of the problems I'm experiencing, I would love to hear from you.

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Which way did you use to add the framework? Did you doubleclick the app in the left bar under Targets and link the library there?