Pip says "I discovered a singular affinity between seeds and corduroys." What is another way of saying the same thing?

The quotation in your question is drawn from Chapter 8 of Great Expectations. Pip is staying with his Uncle Pumblechook overnight before being introduced to Miss Havisham. Pumblechook is a corn chandler (grain retailer) and seedsman. Pip describes his premises and part of the business section of the small town.

In the same early morning, I discovered a singular affinity between seeds and corduroys. Mr. Pumblechook wore corduroys, and so did his shopman; and somehow, there was a general air and flavour about the corduroys, so much in the nature of seeds, and a general air and flavour about the seeds, so much in the nature of corduroys, that I hardly knew which was which. 

What Pip seems to be saying in a humorous, roundabout way is that the corduroys worn by Pumblechook and his shopman are the kind of material that naturally attracts anything small and light such as lint or bits of vegetation, and since both men work with seeds their corduroys are liberally covered with the seeds. In fact, they are so heavily sprinkled with the seeds they trade in that it is hard for Pip to tell whether they are wearing corduroys covered with seeds or seeds covered with bits of corduroy. No doubt the description is intentionally exaggerated for a humorous purpose. 

Corduroys used to mean only corduroy trousers in popular American parlance. Boys often wore corduroys to school and for play before other materials such as denim became more common. But perhaps in Dickens time some men wore corduroy trousers and matching jackets--or even corduroy trousers, jackets and vests. We still occasionally see men wearing corduroy suits, and some corduroy suits may be custom-made by the best tailors. Corduroy suits seem to be especially favored by men such as architects and contractors, who work in their offices and also out in the field on buildings and other jobs under construction.

Corduroy is a thick, durable cotton material with velvety, somewhat fuzzy, ribs or wales. The name has been thought to derive from the French cord du roi, meaning "the king's cords," but the origin is uncertain. The soft, fuzzy ribs or wales would obviously attract such things as seeds in a shop like Pumblechook's. Pip describes the interior of his uncle's shop in the opening paragraph of Chapter 8.

MR. PUMBLECHOOK'S PREMISES in the High-street of the market town, were of a peppercorny and farinaceous character, as the premises of a corn-chandler and seedsman should be. It appeared to me that he must be a very happy man indeed, to have so many little drawers in his shop: and I wondered when I peeped into one or two on the lower tiers, and saw the tied-up brown paper packets inside, whether the flower-seeds and bulbs ever wanted of a fine day to break out of those jails, and bloom.