Double lined gets used a lot in swimwear descriptions, including mine, so here's what it actually means in practice, because it's the single biggest reason a bikini holds its shape or doesn't.
Single lined vs double lined
Most budget swimwear is single lined, one layer of fabric with a bit of stretch. It looks fine standing still, but the moment it's wet and moving, it loses structure fast. Double lined swimwear uses two layers of fabric worked together, which is what actually gives a piece support and shape rather than it just being a layer of stretch that shifts as you move.
Why it matters more once you're actually swimming
On the hanger, you can't always tell the difference. In the water, you can. Double lining is what stops a piece from going see through when wet, what keeps a scoop neckline sitting where it should through a lap of freestyle, and what stops fabric from stretching out of shape after a season of wear.
The lining is doing most of the work. The print is just what you see.
Seamless construction
Every Amali piece is also seamless, which means there's no extra stitching line to catch, dig in, or wear through over time. Paired with double lining, it's what lets a design like Ozoux or Marie move properly with your body instead of against it.
How to tell before you buy
Look for double lining and seamless construction in the product description, and wide or adjustable straps rather than thin strings. If a product page doesn't mention any of that, it's worth asking before you buy.
Every piece in the Ozoux collection and Amali Sets is built this way, tested by me in the ocean before it's ever sold.