Comparing two text files and keeping duplicate lines?

Question

Windows

Hello.
Need a program to compare two text files and save duplicate lines from them to a third file. AXIS: WINDOWS 7
For example.
File_1
01
02
03
File_2
01
04
05
File_3 (result of software operation)
01

Answer 1

duplicate finder

Answer 2

Powershell one-liner

Get-Content file1 + Get-Content file2 | Group | Where {$_.count -gt 1} | Select -ExpandProperty Name | Out-File file3