Code:
import os
print(os.path.join("C:/123\\345", "\\", "folder///filename.bin"))
import pathlib
print(pathlib.PureWindowsPath("C:/123\\345", "\\", "folder///filename.bin"))

Result:
C:\folder///filename.bin
C:\folder\filename.bin

Expected result for Windows:
C:\123\345\folder\filename.bin

The number of slashes should be controlled by the library. Replacing / on \ should also depend on the OS.