diff --git a/src/Microsoft.FeatureManagement/FeatureFilters/Recurrence/RecurrenceEvaluator.cs b/src/Microsoft.FeatureManagement/FeatureFilters/Recurrence/RecurrenceEvaluator.cs
index 5e8b6872..200910ca 100644
--- a/src/Microsoft.FeatureManagement/FeatureFilters/Recurrence/RecurrenceEvaluator.cs
+++ b/src/Microsoft.FeatureManagement/FeatureFilters/Recurrence/RecurrenceEvaluator.cs
@@ -347,7 +347,7 @@ private static int CalculateWeeklyDayOffset(DayOfWeek day1, DayOfWeek day2)
         /// </summary>
         private static List<DayOfWeek> SortDaysOfWeek(IEnumerable<DayOfWeek> daysOfWeek, DayOfWeek firstDayOfWeek)
         {
-            List<DayOfWeek> result = daysOfWeek.ToList();
+            List<DayOfWeek> result = daysOfWeek.Distinct().ToList(); // dedup
 
             result.Sort((x, y) =>
                 CalculateWeeklyDayOffset(x, firstDayOfWeek)
diff --git a/src/Microsoft.FeatureManagement/FeatureFilters/Recurrence/RecurrenceValidator.cs b/src/Microsoft.FeatureManagement/FeatureFilters/Recurrence/RecurrenceValidator.cs
index 230ae674..54b31e16 100644
--- a/src/Microsoft.FeatureManagement/FeatureFilters/Recurrence/RecurrenceValidator.cs
+++ b/src/Microsoft.FeatureManagement/FeatureFilters/Recurrence/RecurrenceValidator.cs
@@ -341,49 +341,40 @@ private static bool TryValidateNumberOfOccurrences(TimeWindowFilterSettings sett
         private static bool IsDurationCompliantWithDaysOfWeek(TimeSpan duration, int interval, IEnumerable<DayOfWeek> daysOfWeek, DayOfWeek firstDayOfWeek)
         {
             Debug.Assert(interval > 0);
+            Debug.Assert(daysOfWeek.Count() > 0);
 
             if (daysOfWeek.Count() == 1)
             {
                 return true;
             }
 
-            DateTime firstDayOfThisWeek = DateTime.Today.AddDays(
-                DaysPerWeek - CalculateWeeklyDayOffset(DateTime.Today.DayOfWeek, firstDayOfWeek));
-
             List<DayOfWeek> sortedDaysOfWeek = SortDaysOfWeek(daysOfWeek, firstDayOfWeek);
 
-            DateTime prev = DateTime.MinValue;
+            DayOfWeek firstDay = sortedDaysOfWeek.First(); // the closest occurrence day to the first day of week
+
+            DayOfWeek prev = firstDay;
 
             TimeSpan minGap = TimeSpan.FromDays(DaysPerWeek);
 
-            foreach (DayOfWeek dayOfWeek in sortedDaysOfWeek)
+            for (int i = 1; i < sortedDaysOfWeek.Count(); i++) // start from the second day to calculate the gap
             {
-                DateTime date = firstDayOfThisWeek.AddDays(
-                        CalculateWeeklyDayOffset(dayOfWeek, firstDayOfWeek));
+                DayOfWeek dayOfWeek = sortedDaysOfWeek[i];
 
-                if (prev != DateTime.MinValue)
-                {
-                    TimeSpan gap = date - prev;
+                TimeSpan gap = TimeSpan.FromDays(CalculateWeeklyDayOffset(dayOfWeek, prev));
 
-                    if (gap < minGap)
-                    {
-                        minGap = gap;
-                    }
+                if (gap < minGap)
+                {
+                    minGap = gap;
                 }
 
-                prev = date;
+                prev = dayOfWeek;
             }
 
             //
             // It may across weeks. Check the next week if the interval is one week.
             if (interval == 1)
             {
-                DateTime firstDayOfNextWeek = firstDayOfThisWeek.AddDays(DaysPerWeek);
-
-                DateTime firstOccurrenceInNextWeek = firstDayOfNextWeek.AddDays(
-                    CalculateWeeklyDayOffset(sortedDaysOfWeek.First(), firstDayOfWeek));
-
-                TimeSpan gap = firstOccurrenceInNextWeek - prev;
+                TimeSpan gap = TimeSpan.FromDays(CalculateWeeklyDayOffset(firstDay, prev));
 
                 if (gap < minGap)
                 {
@@ -413,7 +404,7 @@ private static int CalculateWeeklyDayOffset(DayOfWeek day1, DayOfWeek day2)
         /// </summary>
         private static List<DayOfWeek> SortDaysOfWeek(IEnumerable<DayOfWeek> daysOfWeek, DayOfWeek firstDayOfWeek)
         {
-            List<DayOfWeek> result = daysOfWeek.ToList();
+            List<DayOfWeek> result = daysOfWeek.Distinct().ToList(); // dedup
 
             result.Sort((x, y) =>
                 CalculateWeeklyDayOffset(x, firstDayOfWeek)
diff --git a/tests/Tests.FeatureManagement/RecurrenceEvaluation.cs b/tests/Tests.FeatureManagement/RecurrenceEvaluation.cs
index 4519456e..498abf8a 100644
--- a/tests/Tests.FeatureManagement/RecurrenceEvaluation.cs
+++ b/tests/Tests.FeatureManagement/RecurrenceEvaluation.cs
@@ -291,6 +291,7 @@ public void InvalidTimeWindowAcrossWeeksTest()
                     {
                         Type = RecurrencePatternType.Weekly,
                         Interval = 1,
+                        FirstDayOfWeek = DayOfWeek.Sunday,
                         DaysOfWeek = new List<DayOfWeek>() { DayOfWeek.Tuesday, DayOfWeek.Saturday } // The time window duration should be shorter than 3 days because the gap between Saturday in the previous week and Tuesday in this week is 3 days.
                     },
                     Range = new RecurrenceRange()
@@ -299,7 +300,7 @@ public void InvalidTimeWindowAcrossWeeksTest()
 
             //
             // The settings is valid. No exception should be thrown.
-            RecurrenceEvaluator.IsMatch(DateTimeOffset.UtcNow, settings);
+            Assert.True(RecurrenceValidator.TryValidateSettings(settings, out string paramName, out string errorMessage));
 
             settings = new TimeWindowFilterSettings()
             {
@@ -320,7 +321,49 @@ public void InvalidTimeWindowAcrossWeeksTest()
 
             //
             // The settings is valid. No exception should be thrown.
-            RecurrenceEvaluator.IsMatch(DateTimeOffset.UtcNow, settings);
+            Assert.True(RecurrenceValidator.TryValidateSettings(settings, out paramName, out errorMessage));
+
+            settings = new TimeWindowFilterSettings()
+            {
+                Start = DateTimeOffset.Parse("2024-1-15T00:00:00+08:00"), // Monday
+                End = DateTimeOffset.Parse("2024-1-17T00:00:00+08:00"), // Time window duration is 2 days.
+                Recurrence = new Recurrence()
+                {
+                    Pattern = new RecurrencePattern()
+                    {
+                        Type = RecurrencePatternType.Weekly,
+                        Interval = 1,
+                        FirstDayOfWeek = DayOfWeek.Sunday,
+                        DaysOfWeek = new List<DayOfWeek>() { DayOfWeek.Monday, DayOfWeek.Saturday }
+                    },
+                    Range = new RecurrenceRange()
+                }
+            };
+
+            //
+            // The settings is valid. No exception should be thrown.
+            Assert.True(RecurrenceValidator.TryValidateSettings(settings, out paramName, out errorMessage));
+
+            settings = new TimeWindowFilterSettings()
+            {
+                Start = DateTimeOffset.Parse("2024-1-15T00:00:00+08:00"), // Monday
+                End = DateTimeOffset.Parse("2024-1-17T00:00:01+08:00"), // Time window duration is more than 2 days.
+                Recurrence = new Recurrence()
+                {
+                    Pattern = new RecurrencePattern()
+                    {
+                        Type = RecurrencePatternType.Weekly,
+                        Interval = 1,
+                        FirstDayOfWeek = DayOfWeek.Sunday,
+                        DaysOfWeek = new List<DayOfWeek>() { DayOfWeek.Monday, DayOfWeek.Saturday }
+                    },
+                    Range = new RecurrenceRange()
+                }
+            };
+
+            Assert.False(RecurrenceValidator.TryValidateSettings(settings, out paramName, out errorMessage));
+            Assert.Equal(ParamName.End, paramName);
+            Assert.Equal(ErrorMessage.TimeWindowDurationOutOfRange, errorMessage);
 
             settings = new TimeWindowFilterSettings()
             {
@@ -339,7 +382,7 @@ public void InvalidTimeWindowAcrossWeeksTest()
                 }
             };
 
-            Assert.False(RecurrenceValidator.TryValidateSettings(settings, out string paramName, out string errorMessage));
+            Assert.False(RecurrenceValidator.TryValidateSettings(settings, out paramName, out errorMessage));
             Assert.Equal(ParamName.End, paramName);
             Assert.Equal(ErrorMessage.TimeWindowDurationOutOfRange, errorMessage);
         }