A320
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Last 90 Days 250; Flight Crew Total 15000; Flight Crew Type 3000
Other / Unknown; Situational Awareness
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Airspace Structure; Human Factors
After entering and verifying our cleared route; through a strict enter and read back procedure; we departed for destination. Enroute we were given vectors off course for in-trail spacing to destination. Before being returned to our planned route we discovered that instead of KD45U being in the FMS (clearance waypoint); KD54U was entered instead. Also; another KD54U waypoint was used earlier in the flight plan. The waypoint was corrected and we continued to destination uneventfully after a thorough review of the remainder of the  FMS flight plan.This situation occurred despite a full route review and read back by both pilots prior to departure. These two waypoints are also close enough together to appear to be a valid routing when viewed on the display monitor.I don't know any quick fix for this issue except continuous vigilance. It might be worth reviewing the waypoint numbering system to ensure similar waypoint names are not spaced so closely together. I'll bet this is not the first time this issue has reared its ugly head!
A320 First Officer reports discovering Enroute that NRS waypoint KD45U has been inadvertently entered into the FMC as KD54U. This is detected and corrected before a significant deviation results.