What I have is a sheet with duplicate URL’s in a column and I want to
know what rows the duplicates appear in.

For example:

Row URL

1 www.aaaa.com
2 www.xxxx.com
3 www.aaaa.com
4 www.aaaa.com
5 www.xxxx.com
6 www.ffff.com

What I want to get is:

Row URL Duplicate column rows

1 www.aaaa.com 1,3,4
2 www.xxxx.com 2,5
3 www.aaaa.com 1,3,4
4 www.aaaa.com 1,3,4
5 www.xxxx.com 2,5
6 www.ffff.com 6

Can anyone help?
Cheers
Matt