'判断是否打开了form1，只允许打开一个
Public Form1YN As Boolean

程序代码：

Private Sub Form_Load()
    Form1YN = True
End Sub
Private Sub Form_Unload(Cancel As Integer)
    Form1YN = False
End Sub

程序代码：

Private Sub Command1_Click()
'标志法
Dim p As Form1
If Not Form1YN Then
    Set p = New Form1
    p.Show
Else
    MsgBox "Form1 已打开"
End If
End Sub

程序代码：

Private Sub Command2_Click()
'遍类法
Dim p As Form
For Each p In Forms
    If p.Name = "Form1" Then
        MsgBox "Form1 已打开"
        Exit For
    End If
Next
End Sub