Double Spending
Trying to spend the same coins twice.
Double spending is the attempt to use the same coins in two transactions. Preventing it without a central authority is the core problem a blockchain solves.
Proof of work makes double spending impractical: rewriting a confirmed transaction would mean out-mining the entire honest network, which is prohibitively expensive.
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Pick a coin on the pools overview and open its How to Mine guide for a ready-to-run command.