[BUG] Crash when renaming a file to one that already exists

[EmberHeartshine]

MFR version: 1.0.0.5
Platform: Windows 10 LTSC 1809

Steps to reproduce

1. Select a directory containing files with similar names, ex. demofile1.txt, demofile2.txt, demofile3.txt etc.
2. Set up a process that will rename files to the same name, ex. "remove 1 character from the end of the filename"
3. Click "Go"

Expected behavior
A warning should display before renaming any files

Actual behavior
The first file is renamed, then an unhandled exception occurs.

Other information
See #3 (comment) for full log.

[EmberHeartshine]

See the end of this message for details on invoking 
just-in-time (JIT) debugging instead of this dialog box.

************** Exception Text **************
System.IO.IOException: Cannot create a file when that file already exists.

   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   at System.IO.File.Move(String sourceFileName, String destFileName)
   at MassFileRenamer.frmMain.StripXfromEnd(Int32 strip)
   at MassFileRenamer.frmMain.btnStripX_Click(Object sender, EventArgs e)
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)


************** Loaded Assemblies **************
mscorlib
    Assembly Version: 2.0.0.0
    Win32 Version: 2.0.50727.9054 (WinRelRS5.050727-9000)
    CodeBase: file:///C:/Windows/Microsoft.NET/Framework64/v2.0.50727/mscorlib.dll
----------------------------------------
MassFileRenamer
    Assembly Version: 1.0.0.5
    Win32 Version: 1.0.0.5
    CodeBase: file:///A:/Downloads/MassFileRenamer.exe
----------------------------------------
Microsoft.VisualBasic
    Assembly Version: 8.0.0.0
    Win32 Version: 8.0.50727.9031 (WinRelRS5.050727-9000)
    CodeBase: file:///C:/Windows/assembly/GAC_MSIL/Microsoft.VisualBasic/8.0.0.0__b03f5f7f11d50a3a/Microsoft.VisualBasic.dll
----------------------------------------
System
    Assembly Version: 2.0.0.0
    Win32 Version: 2.0.50727.9042 (WinRelRS5.050727-9000)
    CodeBase: file:///C:/Windows/assembly/GAC_MSIL/System/2.0.0.0__b77a5c561934e089/System.dll
----------------------------------------
System.Windows.Forms
    Assembly Version: 2.0.0.0
    Win32 Version: 2.0.50727.9038 (WinRelRS5.050727-9000)
    CodeBase: file:///C:/Windows/assembly/GAC_MSIL/System.Windows.Forms/2.0.0.0__b77a5c561934e089/System.Windows.Forms.dll
----------------------------------------
System.Drawing
    Assembly Version: 2.0.0.0
    Win32 Version: 2.0.50727.9031 (WinRelRS5.050727-9000)
    CodeBase: file:///C:/Windows/assembly/GAC_MSIL/System.Drawing/2.0.0.0__b03f5f7f11d50a3a/System.Drawing.dll
----------------------------------------
System.Runtime.Remoting
    Assembly Version: 2.0.0.0
    Win32 Version: 2.0.50727.9031 (WinRelRS5.050727-9000)
    CodeBase: file:///C:/Windows/assembly/GAC_MSIL/System.Runtime.Remoting/2.0.0.0__b77a5c561934e089/System.Runtime.Remoting.dll
----------------------------------------
Accessibility
    Assembly Version: 2.0.0.0
    Win32 Version: 2.0.50727.9031 (WinRelRS5.050727-9000)
    CodeBase: file:///C:/Windows/assembly/GAC_MSIL/Accessibility/2.0.0.0__b03f5f7f11d50a3a/Accessibility.dll
----------------------------------------

************** JIT Debugging **************
To enable just-in-time (JIT) debugging, the .config file for this
application or computer (machine.config) must have the
jitDebugging value set in the system.windows.forms section.
The application must also be compiled with debugging
enabled.

For example:

<configuration>
    <system.windows.forms jitDebugging="true" />
</configuration>

When JIT debugging is enabled, any unhandled exception
will be sent to the JIT debugger registered on the computer
rather than be handled by this dialog box.

[EmberHeartshine]

Duplicate files will now have an underscore appended to the filename (before extension).