Comment by 0x000000001

It's not about double spend. It's entirely possible two miners successfully mine a block simultaneously but one of the miners didn't include your transaction in the block they mined. Now there's two competing blockchains and every miner chooses which one is the truth... longest blockchain wins, so whichever gets the most participants is likely to be the final blockchain

Now your transaction may not be included until a future block or be lost forever, though I'm not certain what it takes to be dropped from the mempool.