Mousewheel Events Called Twice

I'm making a program to play video on C#.
I'd like to implement frame-forward playback with mouse wheel, but I'm going to have a case of title.

Immediately after startup, it is called only once, but it is called twice when there is an active control by clicking inside the form.

Events are only available on the main form

this.MouseWheel+=newSystem.Windows.Forms.MouseEventHandler(this.frmMainForm_MouseWheel)

Registering in the form of .

Now

 if (this.ActiveControl!=null)
{
    frameSec = frameMSec/2;
}

If there is an active control like this, I would like to know the fundamental solution rather than this symptomatic solution.

How can I avoid a second call?

Why are they called twice in the first place?