Fix multi dispose scheduler and timer (net-daemon#799)

* Check if already disposed and skip if so

* Add unit test to check dispose functionality

Author: Rick van Dam

src/Extensions/NetDaemon.Extensions.Scheduling.Tests/Scheduling/DisposableSchedulerTest.cs

@@ -78,6 +78,15 @@ public void SchedulePeriodicStopsAfterDisposeOfSubscriber()
         inner.AdvanceBy(TimeSpan.FromMinutes(1).Ticks);
         called.Should().Be(3);
     }
+    
+    [Fact]
+    public void DisposeTwice_NoException()
+    {
+        var (_, disposableScheduler) = CreateScheduler();
+
+        disposableScheduler.Dispose();
+        disposableScheduler.Dispose();
+    }
 
     private (TestScheduler inner, DisposableScheduler disposableScheduler) CreateScheduler()
     {

src/Extensions/NetDaemon.Extensions.Scheduling.Tests/Scheduling/DisposableTimerTest.cs

@@ -0,0 +1,17 @@
+using System.Threading;
+using NetDaemon.Extensions.Scheduler;
+using Xunit;
+
+namespace NetDaemon.Extensions.Scheduling.Tests;
+
+public class DisposableTimerTest
+{
+    [Fact]
+    public void DisposeTwice_NoException()
+    {
+        var timer = new DisposableTimer(CancellationToken.None);
+        
+        timer.Dispose();
+        timer.Dispose();
+    }
+}

src/Extensions/NetDaemon.Extensions.Scheduling/DisposableScheduler.cs

@@ -76,9 +76,11 @@ public IDisposable Schedule<TState>(TState state, DateTimeOffset dueTime, Func<I
     /// <inheritdoc />
     public void Dispose()
     {
-        _isDisposed = true;
-
-        _cancellationTokenSource.Cancel();
-        _cancellationTokenSource.Dispose();
+        if (!_isDisposed)
+        {
+            _cancellationTokenSource.Cancel();
+            _cancellationTokenSource.Dispose();
+            _isDisposed = true;
+        }
     }
 }

src/Extensions/NetDaemon.Extensions.Scheduling/DisposableTimer.cs

@@ -10,6 +10,8 @@ public sealed class DisposableTimer : IDisposable
 {
     private readonly CancellationTokenSource _combinedToken;
     private readonly CancellationTokenSource _internalToken;
+    private bool _disposed;
+    
     /// <summary>
     ///     Constructor
     /// </summary>
@@ -29,8 +31,11 @@ public DisposableTimer(CancellationToken token)
     /// </summary>
     public void Dispose()
     {
-        _internalToken.Cancel();
-        _combinedToken.Dispose();
-        _internalToken.Dispose();
+        if (!_disposed){
+            _internalToken.Cancel();
+            _combinedToken.Dispose();
+            _internalToken.Dispose();
+            _disposed = true;
+        }
     }
 }