In cryptocurrency, double spending is what happens when a digital coin can be spent twice. This obviously would cause the entire system to break ...
source https://www.google.com/url?rct=j&sa=t&url=https://bitsgap.com/blog/double-spending-problem-attack-in-cryptocurrency-explained/&ct=ga&cd=CAIyGmM2Nzk3ODA2ZWViNjVjODI6Y29tOmVuOlVT&usg=AFQjCNHs5wULI-AqaHHD3ScDpBk0TmRteA
