Take a deep breath, let your shoulders down, and exhale. Many sellers have more than one of an item. Is your purchase a unique, one-of-a-kind item, handcrafted, or a rare antique? If it is manufactured, chances are more than one was made. Some sellers put up a multi-item listings, others prefer to sell one at a time, and list another whenever one is sold.
A seller may cancel a purchase, but must ask the buyer to agree to a cancellation, and must refund the purchase price. Have you checked your eBay Messages? How long ago did you pay for your item? Has your seller indicated that your purchase has been shipped? Where is it coming from and what is the estimated delivery date? You are protected by both eBay and PayPal buyer protection. If it doesn't arrive within the estimated delivery time, I would wait another week and then come back here and ask what to do next.