Why AcceptButton is being called twice?

Question

Edit: As Rob said, I asked the question again, with new and correct title here.

I have a form with CancelButton and AcceptButton (named btnCancel and btnOK). And I have some ComboBoxes as input fields.

ComboBoxes prevent my AcceptButton and CancelButton to receive Escape and Enter keys, so I added this code to KeyDown event for all fields:

if (e.KeyData == Keys.Escape)
{
    ComboBox field = (ComboBox)sender;
    if ((field.DropDownStyle == ComboBoxStyle.Simple) || (!field.DroppedDown))
    {
        e.SuppressKeyPress = true;
        btnCancel.PerformClick();
    }
}
else if (e.KeyData == Keys.Enter)
{
    ComboBox field = (ComboBox)sender;
    if ((field.DropDownStyle == ComboBoxStyle.Simple) || (!field.DroppedDown))
    {
        e.SuppressKeyPress = true;
        btnOK.PerformClick();
    }
}

This is the code in Clicked event of OK button:

if (!changesAreSaved)
{
    SaveChangesToNode();
}

List<int> invalidIndices = ValidateAndRefineNodes(true);

if (invalidIndices.Count == 0)
{
    this.DialogResult = DialogResult.OK;
    this.Close();
}
else
{
    if (nodesToEdit.Count > 1)
    {
        trvCategories.SelectedNode = trvCategories.Nodes[invalidIndices[0]];
    }
    FocusOnFirstField(true);
    MessageBox.Show(this, "Enter correct values for all fields before you press OK.", "Cannot Save Information",
        MessageBoxButtons.OK, MessageBoxIcon.Error);
}

Everything is OK but btnOK_Clicked is being called twice only when it shows its MessageBox.

This is Call Stack for first call:

And this is for second:

As you see, second call should not occur at all. In second Call Stack, first btnOK_Click (third line) again calls Fields_KeyDown (second line) from MessageBox.Show(...). How is this possible? I'm confused...

Edit: When I click on OK button, everything is OK. This issue occurs only when I press Enter on my keyboard.