Mach-O linker error when building

Question

I get a Mach-O Linker error when I try to build my app.

ld: 9 duplicate symbols for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)

What have I done wrong?

Answer 1

Deleting derived data solved it

Answer 2

You might have two copies of the same file. Delete one. Check in compile sources under the build phase. You should be able to find a duplicated file.