When you create a sale you have this option:
Additional settings
- Allow items to be claimed — People can permanently claim a free copy while the sale is active
If you tick the block, some people will harvest keys. They will keep the keys to use later, give them away on their own websites, or sell them on eBay - usually as part of a group. of games So you'll have ongoing downloads from time to time (without additional sales) which are using the keys that were harvested.
If you don't tick that box, then people can only get your game while it's on sale, and if they lose the download, they can't claim it again.