copies the input
or the standard input, to the
standard output, comparing adjacent lines.
In the normal case, the second and succeeding copies
of repeated lines are
Repeated lines must be adjacent
in order to be found.
Print unique lines.
Print (one copy of) duplicated lines.
Prefix a repetition count and a tab to each output line.
together with any blanks before each are ignored.
A field is defined as a string of non-space, non-tab characters
separated by tabs and spaces from its neighbors.
characters are ignored.
Fields are skipped before characters.