Counting and numbering duplicates in a list

[Roger Eller]

There are several ways I could approach this, but I'm unsure which way is
best?  I have a list of numbers that 'may' contain duplicates.  I need to
sequence ONLY the duplicates without changing the order the list. If there
is only one, it does not need to be sequenced.

Should I just repeat, and keep the content of line x in a variable, then add
1 to a sequence variable if the number is encountered again?  Is there a
better way?  Simple stuff, I know, but these lists can be really long, and I
want it to process as quickly possible.

12345
12345
12344
12333
10112
12333

must become:

12345-1
12345-2
12344
12333-1
10112
12333-2

˜Roger