If your new duvet cover no longer fits the bed after the first wash, the disappointment is understandable. But there is fairly logical physics behind it.
During weaving and finishing, the yarns are under constant tension. The fabric leaves the mill carrying that tension inside it. When it meets hot water and movement, the fibres relax, swell and try to return to their natural state. The result: shrinkage. In cotton, shrinkage of 3 to 5 per cent is unfortunately still considered normal.
Good manufacturers prevent this with a process called sanforisation. The fabric is pre-shrunk in a controlled way at the mill, so there is little shrinkage left to happen in your home. If the label says "pre-shrunk" or "sanforised", that is a reassurance. The most important thing you can do at home is to be careful with the tumble dryer. Most shrinkage happens not in the wash but in high-heat drying. Drying cottons on low heat, or taking them out while still slightly damp, is the most effective way to keep a product's size.