One second Stan had been peacefully cleaning and polishing his assortment of weapons and watching the recording of "The Duchess Approves" he'd smuggled onboard, and still wasn't ready to tell Ford about.
The next, he, the weapons, the laptop and everything else that was loose enough to do so was levitating into the air, giving him a horrible feeling of déjà vu.
The second thought to cross Stan's mind was to freak out about the fact that Ford was outside and going to float into the air and he might land in the ocean and drown or freeze to death once the gravitational anomaly stopped, or just float so high into the upper atmosphere that he would never come down.
The first thought is...somewhat unprintable.
As soon as gravity reset, he picked himself up and leaped frantically to the window, where he saw that 1) to his relief, Ford was still on the boat, but 2) there was someone else suddenly there, and he had probably been responsible for what just happened, and was therefore a threat to be dealt with in Stan's book.
Fortunately, Stan's punch hadn't hit straight on, or the brass knuckles would have broken his brother's jaw for sure.
As it was, he was going to have a pretty big bruise, and it meant that he crashed into the stranger, who Stan could now see looked like some kind of weird döppelganger of him when he'd been young, so they landed with a crunch in an ungainly heap on the deck.
A little voice in Stan's head muttered that in a way this was payback for when Ford had punched him first thing out of the portal. He ignored it in favor of starting to lunge forward again, before the alternate version of himself pushed Ford off and scooted away, yelling as he pulled himself up, "No, stay away from me!"
Stan advanced anyway, but then his brother was jumping between them again, rubbing his jaw with one hand and saying quickly, "He's right, Stanley! You two need to not come anywhere near each other!"
"Ford, what the heck-!"
"If two versions of the same person from separate dimensions touch, they'll disintegrate not only each other, but the universe they're in!"
Well. That was certainly a good reason. But wait a minute.
"What about when we went to the dimension with all the Mabels? They were all fine."
Ford waved a hand. "It was a dimension that was specifically compatible for all of them to coexist in. It won't work that way out here."
The younger Stan blinked in bewilderment. "You-you've been to other dimensions? But how-?" He peered over the side of the boat, at the sign, then back at them, still looking very confused. "How do you even know about-" Then he glanced down at his side, at some weird science doohickey (it looked a little bit like a laser gun, an hourglass and a mini generator had somehow had a baby) attached to his belt. It was now cracked open and dripping some kind of sparkly multicolored goo that looked a little like it was made of the night sky.
His expression turned to abject horror.
"No, no no no no no no!" Seconds later he was oblivious to both of them, falling to his knees and trying ineffectively to scoop up some of the stuff which was now soaking into the wood of the deck.
"No, I can't-I have to-" As best he could he lifted a handful of the goo, trickling it back into the hourglass part of the gun and trying to hold it shut with his other hand.
Stan glanced at Ford.
"How do we know he's real, and not some kind of hallucination or mind-reading shapeshifter?"
Even if he acted and sounded real, Stan had learned the hard way that sometimes you just couldn't trust your senses.
Ford's eyes widened behind their glasses. "That's a really good point."
"You don't need to sound so surprised."
Ford just elbowed him a little in the ribs. "I need to grab my scanner. Keep an eye on him, and avoid physical contact just in case." He turned and rushed into the cabin.
Stan looked back at the other him. He'd given up trying to grab whatever the stuff was, and was slumped dejectedly in place. And his eyes were raised in the direction Ford had gone, with an expression that was very familiar to the older Stanley in its wistfulness.
If this was a mind-reading shapeshifter or something, he was really good at acting too.
A few crashes and sounds of things being sifted through later, Ford returned brandishing the scanner, and waved it over their guest. Then he examined the screen with a thoughtful frown.
"It registers him as being human, for the most part."
Stan looked at him with a frown. "'For the most part?'"
"There's a few odd bits of DNA mixed here and there. But other than that, it's pretty much the same as when it scans you."
The other Stanley finally pulled himself to his feet, tucking the broken gun back against his hip. In response to Stan's questioning glare, he shrugged. "I've had an interesting life recently."
That certainly sounded like something Stan himself would say. Still...
"How do you know it's not tricking you somehow? Like with an illusion or something?"
"The scanner's programmed to see through those, and besides," Ford reached up and knocked on the side of his skull, letting the metallic sound ring, "this would block out a lot of those kind of attacks from my mind. Please, just trust me."
Another confused gasp from in front of them turned their attention back to the young Stanley.
"...Ford, what happened to your head?" he asked, doing a knocking motion against his own skull.
Ford sighed. "Okay, I think we need a moment to just sit down and get some answers. All of us."
I was planning on actually getting to the explanations and stuff, but it's late and I'm tired, so you're going to have to wait until the next chapter.
Sorry.
It's nothing personal, really.
Also, the Mabel dimension is in the comic "Gravity Falls: Lost Legends," for those not in the know. You should go read the book, or just look up the (probably illegal) version posted on YouTube, if it hasn't been taken down already.