I bought some jeans from Topshop two weeks ago. I threw the reciept and tag/barcode away. I took them back yesterday and the women said i need the tag, so if i took the tag off the same pair of jeans which are still in the shop, would they know?

Yes they would as the stockroom will have assigned each pair of jeans a Sale number so that the manager knows how many she has sold and how many to re-order.These numbers are kept in the stock database an if that Tag gets swiped at the till register, the staff in the shop will know they're different tags from different jeans.