If a friend shows up for coffee wearing “your” teal ballet flats and carrying the white leather handbag you’ve eyed since winter, you might smile to yourself. But what if she paints her dining room the same color as yours and gets a yellow Lab just like your little angel?

We’re all vulnerable to friends who unconsciously forge our signature style — a phenomenon academics call the “chameleon effect.” “Research shows that we naturally tend to behave like those around us,” says John A. Bargh, a professor of psychology at Yale University. “This tendency is stronger for those we like and weaker for those we don’t.”

So what should you do if your style is stolen? “Use humor,” says Melissa Sones, author of Full Frontal Fashion (Plume, $18, www.amazon.com). “Say, ‘Love those shoes,’ and laugh. Let your inflection imply that you notice. And keep telling yourself that imitation is the sincerest form of flattery.” Still, if things get really out of hand — say, your friend steals your hairstyle before moving in on your boyfriend, like the roommate played by Jennifer Jason Leigh in Single White Female — it’s time to call in more than just the fashion police.