The idiom DOUBLE-CROSS means to trick someone.
Centuries ago, when illiteracy was common, and a person was asked to sign a document, they’d do so by inserting an “X” in place of their signature. This was legal. However, the process was often done under pressure (at gunpoint) which meant that the signing party had no intention of honoring the terms of the contract.
Oral lore stated that if a cross was doubled (one written over the other), the first cross was voided thus nullifying the contract. So, to be double crossed was to be duped into a contract or a situation not honored by the other party.