Why is my textBox_enter event (taking focus) called twice here?

Question

this.textBox1.Enter += new System.EventHandler(this.textBox1_Enter);

(...)

int test = 0;
private void textBox1_Enter(object sender, EventArgs e)
{
    ///
    /// update completion from db
    ///

    ++test;
    Log("got focus " + test);
}

I get this result from my log statements:

[03/08/2013 13:56:40]: got focus 1
[03/08/2013 13:56:40]: got focus 2

Why is this function called twice each time I click in my text box?

I already checked: I have only one reference to this function.

Edit:

real function looks more like that

private void textBox1_Enter(object sender, EventArgs e)
{
    // update completion
    List<string> allValues = getValuesFromDb();
    myAutoComplete = new AutoCompleteStringCollection();
    myAutoComplete.AddRange(allValues.ToArray());
    textBox1.AutoCompleteCustomSource = myAutoComplete; /// this line calls enter event again

    ++test;
    Log("got focus " + test);
}

Answer 1

question solved I know why. calling :

textBox1.AutoCompleteCustomSource = myCustomSource;

call enter event again.

So now how to prevent it ?

1) (not working) first solution: move the following piece of code elsewhere

textBox1.AutoCompleteCustomSource =...

not good: autocompletion is not updated

2) (working) put a lock as

 int test = 0;
 bool lockEnter = false;
 private void textBox1_Enter(object sender, EventArgs e)
 {
  if (!lockEnter)
  {
    lockEnter = true;

    // update completion
    List<string> allValues = getValuesFromDb();
    myAutoComplete = new AutoCompleteStringCollection();
    mtAutoComplete.AddRange(allValues.ToArray());
    textBox1.AutoCompleteCustomSource = myAutoComplete;

    ++test;
    Log("update completion " + test);

    lockEnter = false;
  }

}

give result excepted.

Thanks you guys !