VBA - Duplicate rows where celles have values seperated by a comma

Hi, everyone
 
I need you help with a macro I would need to build for an excel file.
 
In my sheet called “Master”, I have multiple columns that could vary. Could be less or could be more.
The first row would always be the column header names.
 
Then I can have up to 3000 rows of data +/-.
 
The objective of the macro is to duplicate rows when a cell from specified columns contain multiple values separated by a comma.
The columns would be identified in a Message Input Box where Column Letters would be separated by a comma as well if more than one as you will see in below picture. So columns to search those values with comma may vary.
 
To add to the complexity, it I select 3 columns that could contain values with comma, it would duplicate the rows making sure that I have a single sample of each values.
 
Ex:

 Private Sub DupRows_Click()
Dim myCols As Variant
myCols = InputBox("Duplicate what columns?" & vbCrLf & "For multiple columns, please separate column letters by a comma.")

With Sheets("Result")
        .UsedRange.Cells.Offset(1, 0).ClearContents
End With


End Sub

 
As you can see in above picture row 2, Column F and M as values separated with Comma. It have 4 different values per cells.
Once the macro runs, it would create 16 identical rows but only put unique values from the strings with comma.
 
For row 3,  in the 3 selected columns, only column C as values separated by comma. It would then duplicate 4 rows but only putting 1 single value per rows.

The result would then be like below:
 
 
The data should be pasted to the sheet called: “Result”. It should not change anything in the sheet called “Master”

I also need to say that the number of values separated by a comma can also vary.
 
How can I do that?
Thank you for your help.


 Duplicate rows no1.xlsm