Lucy's story fills you with a sense of dread, but you try thrusting it away as paranoia. Instead, you focus on Lucy.
"Can you take Lucy back to your place?" you ask Scott.
"What?" he exclaims.
"Well, she needs a place to go, man! I mean, like, if there's, a doppelganger back at her house— We have to take her someplace, and I can't take her back to my house!"
"Well, I can't take her to mine! Besides, we have to get back to—"
But he breaks off at a whimper from Lucy. She is pinned between him and you, and you blush with embarrassment and horrified empathy as she crushes her face into her fists and begins to weep.
Scott gives you a pained look over her head, and pulls her closer to him. "Listen," he tells her in a soothing voice. "Is there anyplace you can go? Anyplace we can take you?" She shakes her head. He looks up at you. "Do you have any ideas?"
You didn't. But in a sudden flash of inspiration, now you do.
--
"It used to be an elementary school," you explain as Scott steers his truck into the small parking lot in back of the old, weather-stained building. It's built of stone and brick, with tall, dark, empty windows and steeply pitched, slate-gray roof. "They closed it a long time ago and turned it into a community center. But it's got a basement and, um—"
You break off as Lucy visibly cringes. "I'm sorry," you tell her. "It's the best I can think of."
"It"—that basement—was also a kind of clubhouse that you and your friends Caleb and Keith made last year. While goofing around at the community center you found an outside door at the bottom of a short staircase. In a fit of curiosity you busted the padlock off and went inside to explore. It led into a large basement, dimly lit by a row of small, grimy windows along one wall. It was dirty and greasy and piled high with ancient desks, conference tables, book shelves, filing cabinets, gym equipment, and even a giant mirror—a filthy junk heap that smelled of dust, old wood, and cold metal. The perfect place for teenage boys to explore and hang out it. 
So after shifting some of the junk around to make an impromptu "fort," you left and returned to put your own lock on the door, and then you and Caleb and Keith spent a couple of months hanging out there on weeknights and weekends, experimenting with whiskey and tobacco. You have not been back since last winter, though, and now that you have returned you doubt that your lock is still on the door. But you figure that you can always break in again.
But Lucy refuses point blank to exit the truck, pointing out that she hasn't got any clothes. And as you can't exactly drive up to your house with a naked girl in the cab – and as you are not looking forward to seeing your dad ... or the thing that might have replaced your dad – you look online for the nearest thrift shop. Fortunately, it turns out there's one only a few blocks away, in Acheson's tiny commercial district, and Lucy ducks down inside the cab while you run inside with every bill that you and Scott are carrying between you.
You come back out with a sleeping bag, a single sheet, a sweatshirt and some sweat pants. (You tried to ask Lucy for her measurements, but she wasn’t willing to share them). Back at the community center, while Lucy dresses in the truck cab, you and Scott investigate the basement. Maybe the padlock on the door is yours and maybe it's a new one, but it doesn't matter. Scott uses the crowbar to wrench it off.
Lucy makes a face when she joins you inside, and you don't blame her. Even without the bad associations of her recent trauma, it would not make a pleasant hang out. But there's no place else to go, and with only a trace of sullenness—which might only be exhaustion, you tell yourself—Lucy says it will do as a temporary hiding place.
Scott pulls you outside as Lucy starts arranging a living space for herself. "She can't just live here," he tells you. "This isn't, you know, a permanent solution."
"I know that! But—"
"We need to hit the professor's house again, get that book away from him. And the sooner we—"
"Yeah, I get it." You rub the back of your head and glance back at the door to the basement. Scott start to say, "You can stay here—" but you interrupt him. "No, I'll go with you. We should pick up some food and water and stuff for her anyway."
But when you go back inside to tell Lucy that you're going to go run some errands, she stops you with a look. "Not before you tell me what you were doing at the professor's," she says.
You and Scott exchange a look. "We weren't trying to break in or anything," you say.
"Bullshit. That's exactly what you were doing." Her expression hardens. "Look," she says, "I'm really grateful to you for getting me out of there, and for putting me up ... here." She glances around with an expression of distaste. "But I still don't know exactly who you are. Or why you're doing this."
Again, you and Scott exchange a look. "We're doing it to help you," you assure her. "It's not like you can go home or go to the police, right?"
"Why not?" she says, setting her jaw.
Your own jaw slackens. "But ... You said there was someone else out there, someone who, uh ... looks like you."
"And you believe me?" Lucy tilts her chin.
You feel your eyes widen. But it's Scott who speaks. "You mean you were bullshitting us?" he says in a hard voice.
"No," she says. "But it does sound like a bullshit story, doesn't it? I mean, even to me it sounds like bullshit, and I'm the one it happened to. So how come you believe me?" She grips the edge of a nearby desk, and her knuckles whiten.
A silence fills the room. You watch Scott out of the corner of your eye as you try figuring out how to answer her question. But again, it's Scott who answers for the both of you.
"Well, here's the thing," he says. "We've got a bullshit story of our own."
--
It comes out very haltingly and in a confused tangle, because there are two stories to tell—your story of finding a book of magic at Arnholm's and Scott's unrelated story of goofing around with some masks he found at work—and you have to back and fill and recap a couple of times before all the facts are laid out, and then you have tell it all again in a more coherent order so Lucy can put all those pieces together. When it is all over and done, she regards you and Scott with a wondering look.
"You're right," she says, "it does sound like bullshit, even more than mine. But it doesn't sound like you're making it up."
"Thanks," you grumble.
But Lucy is staring at Scott. "Mitchell," she murmurs. "I remember reading about that accident. The one that—" She pauses. "That he died in."
Scott's face twists up. "I told you, it wasn't me. It was the real Scott. I—"
Lucy draws up close to him, and puts her arms around him. "I'm so sorry," she says. "It must be horrible."
He embraces her back. "The really hard part of it," he says, "is that I can't even see my family. They think I'm dead, and I can't tell them I'm not, and about what happened."
They hold each other, Lucy snuffling lightly, and Scott squeezing her tightly.
Well, you think, I just lost any chance to score with Lucy. Then you wince at the unworthy thought.
They finally release each other, but even then they continue to hold hands.
"I'm not going to let the same thing happen to you," Scott tells her. "I'm going to get you back to your family, I promise. One way or another—" He looks over at you. "We have to go back over to Blackwell's, try to get that book back. Get anything and everything we can from him."
"Sure." You nod. To Lucy, you add, "And we'll pick you up some stuff. Dinner, water, snacks."
She smiles and says, "Thanks." Then she covers her face with her hands. "Oh God, I'm going to start bawling again," she gasps.
Scott gives you a look, and pulls you up the stairs and outside before you can embarrass Lucy further.
"We'll hit Blackwell's first," he says, "then we'll go pick her up some supplies. Of course, if we're lucky we won't even have to pick her up anything, if we can back in and out with that book."
You nod. But as you get in the truck with him, another thought occurs to you.
Lucy says there's a duplicate of her running around. What if you caught that doppelganger and brought it back to the school, where you could tie it up, question it, find out what it knows? Maybe you could even reverse the impersonation. Blackwell sent a doppelganger out to replace Lucy. Could you send Lucy back in to replace the doppelganger?