If the player uses the same move twice in a row, that's fine. Why not? Nothing in the rules says they can't. But when they make a move, the fiction is already supposed to change right there and then, whether they "succeed" or "fail".
Say we're in your example and the PC is sneaking around. He tries to act under fire and gets a 4, so it's your move, and you want to put someone in a spot. So you say "a guard shows up but doesn't see you." Is that putting them in a spot?
I claim it isn't. It's just embellishing the spot they're already in, right? They were already sneaking around because there was a threat that someone, guard or otherwise, might spot them being somewhere they shouldn't be. They're still somewhere they shouldn't be, and there's still a risk someone might spot them, and the only difference is now we have a particular someone instead of the vague threat of someone. So they can just say "I keep sneaking," sure. Doesn't matter if they succeed or not - the previous failure was already irrelevant.
On the other hand, you could say "A guard shows up and doesn't see you, but she's staring at the door you were heading for." If they try to sneak up to the door and open it while the guard's looking right at it, they aren't acting under fire, they're handing you a golden opportunity on a platter. This time, they can't just keep doing the same thing as before. They have to react to the situation somehow. THAT'S putting someone in a spot.
Or you could say "A guard shows up and starts actively searching for you." Now they can say "Okay, I try to slip past while the guard's back is turned," which I guess is sort of the same thing they were doing before, but you'd be well within your rights to say "They aren't turning their back. They're heading straight for you. Maybe you made a noise, or maybe it's just a lucky guess, but they'll be on you in a moment." Or else you could say "Okay, that's act under fire again," and they ace it and get away. Fine, but the guard is still out there searching for them. The situation has still changed.
Or you could announce offscreen badness instead: "As you duck behind some crates, you hear an explosion nearby. There are shrieks and the sounds of stampeding feet. What do you do?" "Uh... but nobody spots me, right? I just keep sneaking." Sure, if they want! They'll probably come face to face with whatever just happened soon enough. In fact, maybe this time they're not even acting under fire because the guards are too distracted to notice them.
Or remember, you get to make as hard a move as you like, and your job isn't to punish the players or make them lose. Maybe you offer an opportunity without a cost: "You duck behind a stack of crates and your foot hits a trunk on the floor. It has a label on it that says 'guard uniforms'." Maybe you think if the PC poses as a guard, that's tons more interesting than all this sneaking around, and you want to find out what they do if they can move freely around the enemy base. (Or maybe the uniform comes with strings attached! Maybe it's from before the guards' uniforms were redesigned, or maybe there's a nametag with the name of someone important, or maybe just whoever wore it previously had a nasty skin disease.)
The point is, if you make a move, something should change. If something changes, then the fiction has gone somewhere new. If the fiction goes somewhere new, then the die roll wasn't irrelevant, no matter what happens next or if the players "try again".