[Manal]

I earlier had this issue too. But you can refer to this answer for a possible solution to your problem because this helped me. Maybe it would help you too?
Additionally, here is what I did(with reference to the above-mentioned answer).

I copied old source.list to '/' directory as a backup.

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sudo cp /etc/apt/sources.list ~/

Now downloading new source.list and directly making it edited to original file.

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sudo wget "http://pastebin.com/raw.php?i=uzhrtg5M" -O /etc/apt/sources.list

Now updating with new lists.

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sudo apt-get update

Removing extras which had an impact/conflict with another file of the same, so we removed it to avoid the error.

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sudo rm /etc/apt/sources.list.d/ubuntu-extras.list

Now updating again for the last time.

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sudo apt-get update

This should work as it did for me. 
Credits are mentioned.