[lubuntu] How to identify alleged Duplicates in /etc/apt/sources.list

Since the only foolish question is the unasked one, here is my query - others may decide if it is foolish or not.

uname -a

Linux hanraoi-ME051 3.19.0-58-generic #64~14.04.1-Ubuntu SMP Fri Mar 18 19:05:42 UTC 2016 i686 i686 i686 GNU/Linux

The output of

sudo apt-get update

W:  Duplicate sources.list entry http://security.ubuntu.com/ubuntu/  trusty-security/main i386 Packages  (/var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_trusty-security_main_binary-i386_Packages)
W:  Duplicate sources.list entry http://security.ubuntu.com/ubuntu/  trusty-security/restricted i386 Packages  (/var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_trusty-security_restricted_binary-i386_Packages)
W:  Duplicate sources.list entry http://security.ubuntu.com/ubuntu/  trusty-security/universe i386 Packages  (/var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_trusty-security_universe_binary-i386_Packages)
W:  Duplicate sources.list entry http://security.ubuntu.com/ubuntu/  trusty-security/multiverse i386 Packages  (/var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_trusty-security_multiverse_binary-i386_Packages)
W: You may want to run apt-get update to correct these problems

Next, I found in several posts in Ubuntu Forums that I can probably get rid of these "problems" (if problems they are) by editing sources.list. That makes sense, because the report specifically says "Duplicate sources.list entry...", so I back it up, and start editing.

sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
sudo leafpad /etc/apt/sources.list

"http://security.ubuntu.com/ubuntu/ trusty-security/main i386 Packages"

I search for that string with the editor, and get, "Search String not found".
Aoyance comes.

I remove " Packages" from the search string, and search again - and get my friend "Search String not found".
Increment aoyance.

I remove " i386" from the search string, and search again - and get my friend "Search String not found".
Increment aoyance.

I remove "/main", search again, and I find it!
Relief ensues, but not for long.
There is only one match in sources.list, and I thought there was a duplicate here.
Where is it hiding?
Increment aoyance.

I see several possibilities - there may be others.
(1) "sudo apt-get update" is lying, or
(2) it doesn't know what a duplicate is, or
(3) its report format is (uecessarily? deliberately?) obscure, or
(4) I don't recognize a duplicate when I see it.

I'm not about to change something without knowing what I'm doing.

So: How do I identify duplicates in sources.list?