错误：当在窗体上放置多个控件时，引发“主题”属性已由“句点”注册”

如您在下面的代码中看到的，我创建了一个名为Period的自定义控件，该控件从Listbox继承。在其中，我声明了一个名为'Subjects'的只读依赖属性。当在WPF窗口上放置一个句点时，一切运行正常。但是，当我放置多个时，出现标题中提到的错误。

这是期间类：

Public Class Period
Inherits System.Windows.Controls.ListBox

'-------- PROPERTIES --------'
Public ReadOnly Property Subjects() As ObservableCollection(Of Subject)
    Get
        Return Me.GetValue(SubjectsProperty)
    End Get
End Property

Private ReadOnly SubjectsPropertyKey As DependencyPropertyKey = DependencyProperty.RegisterReadOnly("Subjects", GetType(ObservableCollection(Of Subject)), GetType(Period), New FrameworkPropertyMetadata(New ObservableCollection(Of Subject)))
Public ReadOnly SubjectsProperty As DependencyProperty = SubjectsPropertyKey.DependencyProperty

'-------- SUBROUTINES ---------'
Shared Sub New()
    'This OverrideMetadata call tells the system that this element wants to provide a style that is different than its base class.
    'This style is defined in themes\generic.xaml
    DefaultStyleKeyProperty.OverrideMetadata(GetType(Period), New FrameworkPropertyMetadata(GetType(Period)))
End Sub

Public Sub New()
    MyBase.New()

    Me.SetValue(SubjectsPropertyKey, New ObservableCollection(Of Subject))


End Sub


'-------- METHODS ---------'
Public Sub AddSubject(ByRef subject As Subject)
    If Me.CheckForDuplicates(subject) = True Then
        MsgBox("This subject is already present in this period.")
    Else
        Dim SubjectsList As New ObservableCollection(Of Subject)
        SubjectsList = Me.GetValue(SubjectsProperty)
        SubjectsList.Add(subject)

        Me.SetValue(SubjectsPropertyKey, SubjectsList)
    End If
End Sub

Public Sub RemoveSubject(ByRef subject As Subject)
    If Me.CheckForDuplicates(subject) = False Then
        MsgBox("This subject is not present in this period.")
    Else
        Dim SubjectsList As New ObservableCollection(Of Subject)
        SubjectsList = Me.GetValue(SubjectsProperty)
        SubjectsList.Remove(subject)

        Me.SetValue(SubjectsPropertyKey, SubjectsList)
    End If
End Sub

Public Function CheckForDuplicates(ByRef subject As Subject) As Boolean
    Dim Conflict As Boolean

    If Subjects.Contains(subject) Then
        Conflict = True
    End If

    Return Conflict
End Function

Private Sub Period_Initialized(sender As Object, e As EventArgs) Handles Me.Initialized
    Me.ItemsSource = Subjects
End Sub
End Class

这是窗口的代码：

<Grid Background="#FF2B2B2B">
    <local:Period HorizontalAlignment="Left" VerticalAlignment="Top"/>
    <local:Period HorizontalAlignment="Left" VerticalAlignment="Top"/>
</Grid>