Clark Kent. For that matter, Superman's entourage of people with the initials "LL"; Lois Lane, Lex Luthor, Lucy Lane, Lana Lang, Lara Lor-Van, Lori Lemaris, Linda "Supergirl" Lee, Lyla Lerrol, Letitia Lerner, Lee Lambert, Luma Lynai, etc. The first two were coincidence; after that, the writers picked up the theme and ran with it. Also note that Kal-El read backwards also has two L's.
There was a Superman story back in the Silver Age that involved a threat against "someone close to Superman". There was a clue left in the form of two steel girders formed into a giant LL. Superman's reaction was along the lines of "Oh, great, that's real helpful."
Supergirl: While you were gone, I used my super-hearing and heard many Earth-girls' names! I thought of a good one for myself... Linda Lee! How's that? Superman: Er... As good as any! (thinking) Lana Lang was my girlfriend when I was Superboy, and Lois Lane replaced her when I became Superman! By sheer coincidence, she picked the same initials... L. L.!
In Krypton No More, Superman arrested a mob boss called Samuel Simeon.
Talking about Supergirl, one of her best friends back in the Silver Age was Lena Luthor.
Post-Crisis continuity included a Superman fanboy named "Bibbo" Bibbowski, who was a longshoreman and former boxer. (Even though "Bibbo" is a a nickname based on his last name, he still counts because his actual first name is Bo).
Lampshaded in Superman #141: Superman's Return to Krypton, when Kal-El meets Lyla Lerrol, and before being properly introduced he just knows what her initials are on account of her being a beautiful woman who is drawing his attention.
Jor-El: Kal-El, I want you to meet... Kal-El: Don't tell me her name...yet! I'll bet her initials are "L.L."! (thinking) Astonishingly, every girl who ever meant anything to me has alwayd had those initials![...] Jor-El: Of course you know her initials! Who doesn't? Lyla Lerrol is Krypton's most famous emotion-movie actress!
Lampshaded in How Luthor Met Superboy. After meeting Lex Luthor, Superboy realizes he has the same initials as his crush Lana Lang, and feels puzzled about that coincidence.
"Luthor Unleashed": When Luthor starts moonlighting as a supervillain in planet Lexor, its inhabitants call his alter ego the "Mystery Marauder".
In Pre-Crisis comics, Lana's father was called Doctor Lewis Lang.
Smallville continues the double-L theme: Lana Lang's parents are Lewis and Laura Lang, and Lex's dad is Lionel Luthor. Lionel's parents? Lachlan and Eliza. (Several of these were already obscure comics canon.) Lex's mother name was Lillian and in a dream universe his daughter with Lana Lang was named Lily, and his hallucination on a desert island that was obviously himself was called Louis Leery.
Subverted by Lex's actual given name: Alexis/Alexander Joseph Luthor.
In Superman Smashes the Klan, Roberta's full Chinese name is Lan-Shin Lee. At the end of the story, Lois reveals that she gave her a case embroidered with "LL" because she knows Roberta's Chinese name.
