You could use a macro containing the following code:
Dim i As Long
Dim strFind As String
For i = 65 To 122
If i < 91 Or i > 96 Then
strFind = "[" & Chr(i) & "]{2,}"
Selection.HomeKey wdStory
Selection.Find.ClearFormatting
With Selection.Find
Do While .Execute(FindText:=strFind, Forward:=True, _
MatchWildcards:=True, Wrap:=wdFindStop) = True
Selection.Range.Text = Chr(i)
Loop
End With
End If
Next I
but, it would change any replace the repeated characters in words like better, foot, feet, feed with just a single instance of the repeated character so that in the case of these examples, you would end up with beter, fot, fet, fed.