If you click on the button, the event handler starts to process the logic; if the user clicks on the button again, then, the processing will start once again.
To overcome this issue, when the user clicks on the button, disable the button until the processing is complete. Once the processing of the button event is complete, then enable the button once again.
ex.
<asp:Button ID="btnUploadFile" runat="server" Text="Upload" class="BTN_Orange" OnClick="btnUpload_Click"
UseSubmitBehavior="false" OnClientClick="this.disabled = true; this.value = 'Uploading...';"/>