What is double spending in blockchain?

I really need someone to break this down for me. Seriously.

Last Thursday, I was trying to pay my roommate back for a late-night pizza using a tiny sliver of Litecoin. Naturally, my awful rural Wi-Fi choked exactly when I smashed the send button—so I immediately panicked and hammered the button a second time.

That minor heart attack led me straight into a massive late-night internet rabbit hole frantically searching: What is double spending in blockchain?

It didn't actually charge my wallet twice. Thank goodness.

But the sheer panic of almost vaporizing my crypto got me obsessing over how the network functions as a totally decentralized traffic cop. From my messy weekend research into what is double spending in blockchain, I gather it’s basically the virtual equivalent of trying to hand the exact same physical twenty-dollar bill to two different cashiers at precisely the same split second. Obviously, physical cash organically prevents that kind of cheating. You hand the paper over, and boom—it's gone.

Digital money, though? It's just raw computer data (which a clever thief could theoretically copy and paste indefinitely). That completely messes with my head.

My exact dilemma regarding what is double spending in blockchain

Here is where my brain completely stalls out.

• If a random node validates my first pizza payment, how does the rest of the global swarm instantly detect and kill the ghost duplicate?
• Are those infamous 51% attacks the sole avenue where double spending in blockchain actually succeeds nowadays?

For any fellow rookies stumbling around forums wondering what is double spending in blockchain, here is my highly informal, actionable cheat sheet (veterans, please rip this apart if I'm completely off base!):

Does the mempool just automatically reject the second attempt before it even gets verified? I'd genuinely love a highly practical, real-world explanation of this weird mechanic. Skip the terrifying cryptographic algebra, please!