I am wondering about the following scenario when you allow anything to be entered, but do require validation: what if I entered "myaddress@mysite.org, someone-e

I am wondering about the following scenario when you allow anything to be entered, but do require validation: what if I entered "myaddress@mysite.org, someone-else@elsewhere.org". The mail will be sent to both addresses, and if I'm faster than the other guy (probably, because I'm expecting the email), I can basically sign someone else up for whatever it is. Okay, it's trivially easy to find out that I did it (but I could use a throwaway address), but you may want to prevent this scenario anyway to prevent harassment.