Running sudo apt-get update from a terminal is likely to reveal more information about which is coming from the untrusted source. This generally just means that you don't have the key for that repo yet.
If you are sure that the PPA is one you are happy to use then you can add the key with a terminal
gpg --keyserver keyserver.ubuntu.com --recv 36FD5529 28949509
gpg --export --armor 36FD5529 28949509 | sudo apt-key add -
Then update
sudo apt-get update
Hope that helps
If you want to add PPA's in future you can use the add-apt-repository command, this will get the keys as well for you.
sudo add-apt-repository ppa:user/ppa-name
As far as the duplicate is concerned I would check your sources - you can open the file for editing as root and check for duplicates.
Backup the file first
sudo cp /etc/apt/sources.list /etc/apt/sources/list.bak
Then
gksudo gedit /etc/apt/sources.list
Check for exact duplicates and put an # at the beginning of the line, save, exit and apt-get update.
Edit : Follow this link to fix the unmet dependency problem.
Go to your Terminal ( Ctrl+Alt+T ) and type the following commands :
sudo apt-get update
sudo apt-get install ubuntu-restricted-extras
sudo apt-get install vlc
This will install vlc, it's dependencies and you should be able to play videos without any problem.
Best Answer
You need to ensure your software sources are enabled for Restricted Extras and Multiverse.
Open the Software Centre from Unity then select Edit -> Software Sources.
In the dialogue box that opens ensure you have the following selected;
Once this is done close the box and search for VLC in the Software Centre, you will then be able to install it.