fix: resolve duplicate job execution due to timer issues

Author: linkdotnet

CHANGELOG.md

@@ -6,6 +6,10 @@ All notable changes to **NCronJob** will be documented in this file. The project
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed duplicate job execution caused by system timers firing slightly before the scheduled cron time, resulting in the same cron slot being scheduled again. Reported by [@RShergold](https://github.com/RShergold) in [#327](https://github.com/NCronJob-Dev/NCronJob/issues/327). Fixed by [@linkdotnet](https://github.com/linkdotnet).
+
 ## [v4.10.0] - 2026-03-30
 
 ### Added

src/NCronJob/Scheduler/JobWorker.cs

@@ -110,7 +110,7 @@ private async Task DispatchJobForProcessing(
 
             if (!nextJob.IsOneTimeJob)
             {
-                ScheduleJob(nextJob.JobDefinition);
+                ScheduleJob(nextJob.JobDefinition, nextRunTime);
             }
 
             shouldReleaseSemaphore = false;
@@ -189,15 +189,23 @@ private bool CanStartJob(JobDefinition jobEntry)
     private void UpdateRunningJobCount(string jobFullName, int change) =>
         runningJobCounts.AddOrUpdate(jobFullName, change, (_, existingVal) => Math.Max(0, existingVal + change));
 
-    public void ScheduleJob(JobDefinition job)
+    public void ScheduleJob(JobDefinition job, DateTimeOffset? lastScheduledRunTime = null)
     {
         if (!job.IsEnabled)
         {
             return;
         }
 
         var utcNow = timeProvider.GetUtcNow();
-        var nextRunTime = job.GetNextCronOccurrence(utcNow);
+
+        // When rescheduling after a job fires, the timer may have triggered slightly
+        // before the scheduled time. Using utcNow directly could return the same cron
+        // slot again, causing duplicate execution. Using the later of utcNow and the
+        // last scheduled run time guarantees we always advance past the fired slot.
+        var baseTime = lastScheduledRunTime.HasValue && lastScheduledRunTime.Value > utcNow
+            ? lastScheduledRunTime.Value
+            : utcNow;
+        var nextRunTime = job.GetNextCronOccurrence(baseTime);
 
         if (!nextRunTime.HasValue)
         {