If you are behind and buy a second or third dorans it might help you winning your lane or at least avoid falling behind. If you can make use of those items to farm a little bit more (around 10-15 cs) or even kill the enemy once it is definately worth it.

Do doubled blades work?

It is definitely worth it if you don't have enough gold for components for your items, and don't want a long sword. Occasionally I'll stack 3-4 as Lucian mid and I feel strong but usually just 1-2. It gives you damage and durability, but they don't have a good resale value.

Why do people buy double Dorans blade?

Buying double dorans allows you to be a huge lane bully. Some ADCs will build triple dorans blades which will allow them to pretty much stay in lane as long as they want. You got an early lead and go back to shop. You don't have enough to purchase your expensive items but want to keep your lane dominance.