Repeater control events conflict

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C#3.5

ASP.NET4

<asp:Repeater runat="server" ID="RpUserList">
   <itemtemplate>

    <td>
  <asp:Label ID="lblRpuser_EmpName" runat="server" Text='<%#Eval("EmpName")%>'>

  </td>

  <td>
 <asp:CheckBox ID="Chkuser_Active" Checked='<%#Eval("Active") %>' OnCheckedChanged="Chkuser_Active_CheckedChanged"   AutoPostBack="true" runat="server" />
  </td>

  <td>
 <asp:ImageButton ID="imgevaledit" OnClick="imgevaledit_onclick" Width="16px" Height="16px" ImageUrl="~/App_Themes/SGERP/image/pencil.png" runat="server" />

  </td>
   </itemtemplate>

This is my repeater. My problem is when I click the button(imgevaledit) from the repeater first it fire the 'Chkuser_Active_CheckedChanged' event
and then 'imgevaledit_onclick'.

protected void Chkuser_Active_CheckedChanged(object sender, EventArgs e)
{
    if (sender != null)
    {

    ...............
    }
 }

This is my check event and also I checked 'sender!=null' even though check change if block get executed when button click.
Please help me to solve this issue. And give any idea why checkbox event is fired when button click.