[Tutor] Finding duplicates entry in file

[Ken G.]

What is a method I can use to find duplicated entry within a sorted 
numeric file?  

I was trying to read a file reading two lines at once but apparently, I 
can only read one line at a time.  Can the same file be opened and read 
two times within a program?

For example, a file has:

1
2
2
3
4
4
5
6
6

The newly revised file should be:

1
2
3
4
5
6

Again, thanking the group for their input,

Ken