Change Event Based On Two Other Cell Values

myColumnOne = Range("NPN").Column 'this is column B, NPN is a Named Range of B1
myColumnTwo = Range("NPCH").Column 'this is column E, NPCH is a Named Range of E1

using these variables I want to say

When data is entered into any cell in myColumnOne
first check to see if this same data already exists in myColumnOne
if it does then check to see if in the row where the data already exists, if the corresponding cell in myColumnTwo ISBLANK then MsgBox

if the corresponding cell in myColumnTwo is not blank, allow the data to be entered.

Example:
Col B….Col E
ABC…..xxx
XZY….........