brew install -v io

output

-> % brew install -v io
/usr/local/bin/git
==> Cloning https://github.com/stevedekorte/io.git
Updating /Users/developer/Library/Caches/Homebrew/io--git
git fetch https://github.com/stevedekorte/io.git
From https://github.com/stevedekorte/io
 * branch            HEAD       -> FETCH_HEAD
git checkout-index -a -f --prefix=/private/tmp/homebrew-io-HEAD-DnhH/
==> Downloading patches
==> Patching
/usr/bin/patch -f -p1 -i 001-homebrew.diff
patching file addons/Image/CMakeLists.txt
==> cmake .. -DCMAKE_INSTALL_PREFIX='/usr/local/Cellar/io/HEAD' -DCMAKE_BUILD_TYPE=None -Wno-dev
cmake .. -DCMAKE_INSTALL_PREFIX='/usr/local/Cellar/io/HEAD' -DCMAKE_BUILD_TYPE=None -Wno-dev 
-- The C compiler identification is GNU
-- The CXX compiler identification is GNU
-- Checking whether C compiler has -isysroot
-- Checking whether C compiler has -isysroot - yes
-- Checking whether C compiler supports OSX deployment target flag
-- Checking whether C compiler supports OSX deployment target flag - yes
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Checking whether CXX compiler has -isysroot
-- Checking whether CXX compiler has -isysroot - yes
-- Checking whether CXX compiler supports OSX deployment target flag
-- Checking whether CXX compiler supports OSX deployment target flag - yes
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Configuration set to: None
-- Found AsyncRequest headers: /usr/include/aio.h
-- GMP libs: GMP_LIBRARIES-NOTFOUND GMPXX_LIBRARIES-NOTFOUND
-- Could NOT find GMP (missing:  GMP_INCLUDE_DIR GMP_LIBRARIES) 
-- checking for one of the modules 'libffi'
-- Could NOT find FFI (missing:  FFI_INCLUDE_DIRS FFI_LIBRARIES) 
-- checking for one of the modules 'glib-2.0'
-- Found GLib2: glib-2.0;intl /usr/local/Cellar/glib/2.28.7/include/glib-2.0;/usr/local/Cellar/glib/2.28.7/lib/glib-2.0/include;/usr/local/Cellar/gettext/0.18.1.1/include;/usr/local/Cellar/glib/2.28.7/include
-- Looking for include files HAVE_GLIB_GREGEX_H
-- Looking for include files HAVE_GLIB_GREGEX_H - found
-- checking for one of the modules 'cairo'
-- Found Curses: /usr/lib/libcurses.dylib 
-- Found EDIT: /usr/lib/libedit.dylib
-- Found Fnmatch headers: /usr/include/fnmatch.h
-- Found Freetype: /usr/X11R6/lib/libfreetype.dylib 
-- Could NOT find GLFW (missing:  GLFW_LIBRARY GLFW_INCLUDE_DIR) 
-- Found ZLIB: /usr/lib/libz.dylib
-- Found PNG: /usr/X11R6/lib/libpng.dylib 
-- Could NOT find TIFF (missing:  TIFF_LIBRARY TIFF_INCLUDE_DIR) 
-- Could NOT find JPEG (missing:  JPEG_LIBRARY JPEG_INCLUDE_DIR) 
-- Found Iconv: /usr/lib/libiconv.dylib
-- checking for module 'libxml-2.0'
--   found libxml-2.0, version 2.7.3
-- Found LibXml2: /usr/lib/libxml2.dylib 
-- Found MYSQL: /usr/local/lib/libmysqlclient.dylib
-- Found PythonLibs: /usr/lib/libpython2.7.dylib 
-- Found Readline: /usr/lib/libreadline.dylib
-- checking for module 'libpcre'
--   found libpcre, version 8.12
-- Found PCRE: /usr/local/Cellar/pcre/8.12/include 
-- checking for one of the modules 'sqlite>=3;sqlite3'
-- Found OpenSSL: /usr/lib/libssl.dylib;/usr/lib/libcrypto.dylib 
-- Found Event: /usr/local/lib/libevent.dylib
-- Found Syslog headers: /usr/include/syslog.h
-- Looking for include files CMAKE_HAVE_PTHREAD_H
-- Looking for include files CMAKE_HAVE_PTHREAD_H - found
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE 
-- Found TokyoCabinet: /usr/local/lib/libtokyocabinet.dylib
-- Found ZLIB: /usr/lib/libz.dylib
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp/homebrew-io-HEAD-DnhH/io-build
==> make install
make install 
Scanning dependencies of target copy_basekit_headers
[  0%] Copying files: /tmp/homebrew-io-HEAD-DnhH//tmp/homebrew-io-HEAD-DnhH/libs/basekit/source/*.h to /tmp/homebrew-io-HEAD-DnhH/io-build/_build/headers
[  0%] Built target copy_basekit_headers
Scanning dependencies of target copy_coroutine_headers
[  0%] Copying files: /tmp/homebrew-io-HEAD-DnhH//tmp/homebrew-io-HEAD-DnhH/libs/coroutine/source/*.h to /tmp/homebrew-io-HEAD-DnhH/io-build/_build/headers
[  1%] Built target copy_coroutine_headers
Scanning dependencies of target copy_garbagecollector_headers
[  1%] Copying files: /tmp/homebrew-io-HEAD-DnhH//tmp/homebrew-io-HEAD-DnhH/libs/garbagecollector/source/*.h to /tmp/homebrew-io-HEAD-DnhH/io-build/_build/headers
[  1%] Built target copy_garbagecollector_headers
Scanning dependencies of target copy_iovm_headers
[  1%] Copying files: /tmp/homebrew-io-HEAD-DnhH//tmp/homebrew-io-HEAD-DnhH/libs/iovm/source/*.h to /tmp/homebrew-io-HEAD-DnhH/io-build/_build/headers
[  1%] Built target copy_iovm_headers
Scanning dependencies of target io2c
[  1%] Building C object libs/iovm/tools/CMakeFiles/io2c.dir/io2c.c.o
Linking C executable ../../../_build/binaries/io2c
[  1%] Built target io2c
Scanning dependencies of target coroutine
[  1%] Building C object libs/coroutine/CMakeFiles/coroutine.dir/source/context.c.o
[  2%] Building C object libs/coroutine/CMakeFiles/coroutine.dir/source/Coro.c.o
[  2%] Building C object libs/coroutine/CMakeFiles/coroutine.dir/source/asm.S.o
Linking C shared library ../../_build/dll/libcoroutine.dylib
[  2%] Built target coroutine
Scanning dependencies of target basekit
[  2%] Building C object libs/basekit/CMakeFiles/basekit.dir/source/BStream.c.o
[  2%] Building C object libs/basekit/CMakeFiles/basekit.dir/source/BStreamTag.c.o
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[  4%] Building C object libs/basekit/CMakeFiles/basekit.dir/source/Date.c.o
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[  4%] Building C object libs/basekit/CMakeFiles/basekit.dir/source/DynLib.c.o
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[  5%] Building C object libs/basekit/CMakeFiles/basekit.dir/source/List.c.o
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[ 12%] Building C object libs/basekit/CMakeFiles/basekit.dir/source/utf_convert.c.o
Linking C shared library ../../_build/dll/libbasekit.dylib
ld: duplicate symbol _List_size in CMakeFiles/basekit.dir/source/Stack.c.o and CMakeFiles/basekit.dir/source/List.c.o for architecture x86_64
collect2: ld returned 1 exit status
make[2]: *** [_build/dll/libbasekit.dylib] Error 1
make[1]: *** [libs/basekit/CMakeFiles/basekit.dir/all] Error 2
make: *** [all] Error 2
==> Exit Status: 2
http://github.com/mxcl/homebrew/blob/master/Library/Formula/io.rb#L24
==> Environment
HOMEBREW_VERSION: 0.8
HEAD: bd4fdb650642030d2beb295f8ae35def6125028f
HOMEBREW_PREFIX: /usr/local
HOMEBREW_CELLAR: /usr/local/Cellar
HOMEBREW_REPOSITORY: /usr/local
HOMEBREW_LIBRARY_PATH: /usr/local/Library/Homebrew
Hardware: dual-core 64-bit penryn
OS X: 10.7
Kernel Architecture: x86_64
Ruby: 1.8.7-249
/usr/bin/ruby => /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
Xcode: 4.1
GCC-4.0: N/A 
GCC-4.2: build 5666 
LLVM: build 2335 
MacPorts or Fink? false
X11 installed? true
==> Build Flags
CC: /usr/bin/cc => /usr/llvm-gcc-4.2/bin/llvm-gcc-4.2
CXX: /usr/bin/c++ => /usr/llvm-gcc-4.2/bin/llvm-g++-4.2
LD: /usr/bin/cc => /usr/llvm-gcc-4.2/bin/llvm-gcc-4.2
CFLAGS: -O3 -march=core2 -msse4.1 -w -pipe
CXXFLAGS: -O3 -march=core2 -msse4.1 -w -pipe

Error: Failed executing: make install 
Please report this bug: https://github.com/mxcl/homebrew/wiki/Checklist-before-filing-a-new-issue
These existing issues may help you:
https://github.com/mxcl/homebrew/issues/#issue/5254
https://github.com/mxcl/homebrew/issues/#issue/5428
https://github.com/mxcl/homebrew/issues/#issue/5445
https://github.com/mxcl/homebrew/issues/#issue/5597
https://github.com/mxcl/homebrew/issues/#issue/5603
https://github.com/mxcl/homebrew/issues/#issue/5970
https://github.com/mxcl/homebrew/issues/#issue/6043
https://github.com/mxcl/homebrew/issues/#issue/6120
https://github.com/mxcl/homebrew/issues/#issue/6473

Also try:
  `brew doctor` to check your setup for common problems.
  `brew missing` to check installed packages for missing deps.