Why One Shouldn't Forget He's a Twin:
A/N: MWAHAHA! You might like this one.
"Luigi," Mario called to his brother, "I'm going for a walk."
"W-where are you going?" Luigi asked, all too quickly.
"To Peach's castle," Mario replied.
"I-I'll come with you!" Luigi offered, and before Mario could protest, he had grabbed his hat, slamming in his head backwards, and rushed out the door. Mario scratched his head, but followed, confused.
"Are you all right, Luigi?" Mario asked, adjusting Luigi's hat for him. "You seem jumpy."
"J-jumpy?" Luigi cried, his voice an octave higher. He cleared his throat. "I mean, jumpy? No, no, I'm not jumpy. Just restless." He cleared his throat again. "How about we go visit Toad?" he suggested.
"Okay," Mario shrugged. "There's plenty of time to visit Peach later."
"You're right. Plenty of time. In fact, let's visit Toadette as well. And Yoshi, and, and... Hey, why don't we visit all our friends before visiting Peach!"
So poor Mario was dragged all over Toad Town, visiting friends, acquaintances, and ever complete strangers, until that evening.
"Oh, no!" Mario groaned. "No time to visit Peach now!" He crossed his arms, and glared at Luigi, but he didn't notice.
"Plenty of time," Luigi said. "Let's-a go!" He grabbed his brother's hand, and dragged him off to Peach's castle.
Toads worth opened the door, and smiled.
"Good work, Master Luigi," he said. "Come on in, gentlemen." Luigi shoved Mario through the door. The room was dark, and he couldn't see a thing. He bumped into something.
"OOF!" he cried. Then the lights all came in at once, and the room was full of people.
"HAPPY BIRTHDAY, MARIO AND LUIGI!" they exclaimed.
"M-me?" Luigi mumbled. "But I thought this was Mario's party."
"It's yours, too, silly," Daisy laughed. "Have you forgotten? You and Mario are twins!"
"And you were so busy keeping Mario away for the day, you had absolutely no idea this was a party for you, too," Peach added. Luigi face-palmed as Mario laughed.
"Well, what are we waiting for?" Toadette asked impatiently. "Let's have some cake!"