Zune has a share-once restriction which only allows you to share the song only once with your friend using WiFi.

Now with this small hack, you and your friend can share that same song again and again as many times as you like. So you and your friend both has to do this:

Steps:

1. Connect your Zune to your PC and start the Zune software.
2. Open My Computer. Click on Zune link.
3. Open Storage inside Zune link
4. Delete the files with the name of the person who transferred songs to you.
5. Ask your friend to do the same.

Now you can transfer the same song again and again.
So enjoy.