Does homebrew store tar.gz after package installation is complete?
Yes! The latest version of homebrew stores the artifacts used to install the packages in /Library/Caches/Homebrew:

&gt; ls /Library/Caches/Homebrew
Casks                                                 gnutls-3.3.15.yosemite.bottle.tar.gz                  libusb-1.0.19.yosemite.bottle.1.tar.gz                ppl011-0.11_1.yosemite.bottle.tar.gz
Formula                                               gpg-agent-2.0.27.yosemite.bottle.2.tar.gz             libusb-compat-0.1.5.yosemite.bottle.1.tar.gz          protoc-square-2.6.1.square.1.2.yosemite.bottle.tar.gz
brew-cask--git                                        isl-0.14.1.yosemite.bottle.3.tar.gz                   maven-3.3.3.tar.gz                                    pth-2.0.7.yosemite.bottle.1.tar.gz
cloog-0.18.3.tar.gz                                   isl011-0.11.1.yosemite.bottle.tar.gz                  mpfr-3.1.2-p11.yosemite.bottle.tar.gz                 qemu-2.3.0.yosemite.bottle.tar.gz
cloog018-0.18.0.yosemite.bottle.tar.gz                libassuan-2.2.1.yosemite.bottle.tar.gz                nasm-2.11.08.yosemite.bottle.tar.gz                   sqlite-3.8.10.2.yosemite.bottle.tar.gz
colordiff-1.0.13.yosemite.bottle.tar.gz               libgcrypt-1.6.3.yosemite.bottle.tar.gz                nettle-2.7.1.yosemite.bottle.1.tar.gz                 um-1.8.6.tar.gz
dirmngr-1.1.1_1.yosemite.bottle.2.tar.gz              libgpg-error-1.19.yosemite.bottle.tar.gz              p2-0.0.5.1.yosemite.bottle.tar.gz                     vim-7.4.712.tar.gz
gcc-5.1.0.yosemite.bottle.2.tar.gz                    libksba-1.3.3.yosemite.bottle.tar.gz                  packer-0.7.5.yosemite.bottle.tar.gz                   virtualbox-4.3.26-98988.dmg.incomplete
glib-2.44.1.yosemite.bottle.tar.gz                    libmpc-1.0.3.yosemite.bottle.tar.gz                   pcre-8.37.yosemite.bottle.tar.gz                      virtualbox-4.3.28-100309.dmg
gmp-6.0.0a.yosemite.bottle.tar.gz                     libpng-1.6.17.yosemite.bottle.tar.gz                  pinentry-0.9.4.yosemite.bottle.tar.gz
gnupg2-2.0.28.yosemite.bottle.tar.gz                  libtasn1-4.5.yosemite.bottle.tar.gz                   pixman-0.32.6.yosemite.bottle.1.tar.gz


If you interrupted an installation homebrew will resume downloading the artifacts if they don't match the SHA values specified in the recipe for the package. If they SHAs match, it won't redownload them. You won't ever end up with two copies of the same artifact on disk. If it needs to re-download, it'll delete the existing artifact first.