awk 'l[$0]++{d++}END{printf("%d/%d\n",d,NR)}' file.txt

awk '		# Run awk with the following script...
l[$0]++ {	# Set array l[] indexed by the contents of the current input line to
		# the number of times this line has been seen so far and return the
		# number of times this line had been seen before this line.  If the
		# value returned is not zero and is not the empty string, execute
		# the commands in this section.  (This will happen any time this line
		# has been seen before.)

	d++	# Increment the number of duplicates seen.
}
END {		# After all lines have been read from all input files given to
		# this invocation of awk, run the commands in this section.

	printf("%d/%d\n, d, NR)	# Print the number of duplicates seen and
				# the Number of Records read from all of the input files
				# given to this invocation of awk.
}' file.txt	# End the script and specify the input file(s) to be processed by this
		# invocation of awk.