can not change double to single dash

[killerrex]

Please put your code in the [python][/python] markers or all the indentation is lost...

Notice that your statement (the syntax highlighting does not like your profuse use of the backslash...)

>>> "\\".replace("\\\\","\\")
'\\'

is equivalent to:

>>> "A".replace("AA", "A")
'A'

And there is obvious that in a string of 1 char there will be 0 replacements of a 2 chars group.
Your expression works when the input has 2 or more backslashes:

>>> bs = '\\'
>>> s = bs * 8
>>> s
'\\\\\\\\\\\\\\\\'
>>> len(s)
8
>>> s.replace("\\\\", "\\")
'\\\\\\\\'
>>> len(_)
4