Air carrier CRJ900 First Officer reported receiving a master warning triple chime and Left Bleed Duct warning during boarding. Maintenance technicians found a leak in the air conditioning and pressurization system.

Narrative

While boarding at the gate to operate Aircraft X from ZZZ to ZZZ1; the Captain and I received a master warning triple chime aural alert saying Left Bleed Duct. A red L BLEED DUCT EICAS warning was displayed on ED1; and the left hand portion of the ECS (Environmental Control System) page showed red for 30 seconds. After 30 seconds; the EICAS warning message was replaced by an amber caution message that said L BLEED DUCT; the isolation valve closed; and the air conditioning was cut off from the APU. Had this event happened in the air; the aircraft would have become completely depressurized after 30 seconds of receiving the master warning if measures were not taken to transfer the bleed air source prior to that 30 seconds. Contract maintenance at ZZZ found a leak in a part of the distribution ducting that provides the air conditioning and pressurization. I am very concerned about the overall safety of this aircraft. This aircraft has apparently had a history of maintenance issues specifically regarding aircraft air conditioning and pressurization. This history information was not relayed to the Captain or myself; and the situation could have easily been made worse had the leak led to a depressurized condition at cruise. Thankfully; we discovered this issue during pre-flight when we received the master warning. However; I am concerned that this aircraft may again have a malfunction that could create a safety hazard in flight.