'trying to overwrite file already in shotwell-common package' while installing shotwell from the PPA

Try this command on terminal,

sudo dpkg -i --force-overwrite /var/cache/apt/archives/shotwell_0.15.1-1~saucy1_i386.deb

This error occurs when some file was moved from one package to another in a newer version.

In your case dpkg trying to extract shotwell.svg file from /var/cache/apt/archives/shotwell_0.15.1-1~saucy1_i386.deb to /usr/share/icons/hicolor/24x24/apps/ directory.But there was already a shotwell.svg file in that directory.So this type of error occurs.

By running dpkg -i --force-overwrite command,the previously located file shotwell.svg was forcely overwrited by the file from the /var/cache/apt/archives/shotwell_0.15.1-1~saucy1_i386.deb while installing.

Here is the source.

Try these commands,

sudo dpkg -r shotwell-common
sudo apt-get install shotwell

apt-get may or may not work in this case of removing shotwell-common package.