I've done this a few times with those cheapie fashion leggings... You know, the ones that cost like $5 or $10 a pair.
See a cool print, and gotta have it!
At some point down the road, though, I realize I had purchased the same exact pair in the past. Oops!
At least they're pretty inexpensive, anyway. And obviously I must really like them.
I've also caught myself ahead of time for a few of them. "Ooh, I like that one! Oh, wait... This looks familiar. Better check to make sure I don't already own it."
And... turns out I already do.
I usually buy these on eBay, from a few sellers. Luckily, I have quick & easy access to my records there from the past 3 years, so I can catch it ahead of time. Saved a few bucks that way, heh.
To be fair to myself, though, I do own something like at least 250 pairs of leggings, no joke. So, bound to happen every now & then.
Anyway, like some others here, I also sometimes buy duplicates of things *intentionally*. Like, same exact style, size & color. For example, I do this with two particular black long-sleeve fitted tops, which I wear a lot. Hey, if you find something you *really* like? You're bound to wear them often.
So, when I see that they go on sale, and I have the budget for it? Stock up! Because at some point, they'll either discontinue them, or eventually change them enough where it's no longer the same thing, ya know?
Finally, I also do the duplicate thing -- but with different colors. Say, a particular hoodie that you already know you really like? Get 'em in a variety of colors, why not? The more, the merrier!
