###Excel VBA, 110 bytes
This is my first entry to CodeGolf so I hope this is ok.
You enter the input word in A1 and then the letters to be replaced in B1 and the resulting word is displayed in a message box.
w = Cells(1, 1)
l = Cells(2, 1)
q = Len(w)
For i = 1 To q
x = Left(w, 1)
R = R + x
If InStr(l, x) > 0 Then
R = R + x
End If
w = Right(w, q - 1)
Next
MsgBox R