php - Validating Winners -

i`m managing online trading game system. require following fields upon registration:

• email
• username
• password
• sex
• phone number
• name , surname

and users ip upon registration.

the point imposed written terms , conditions on site: 1 user can have 2 usernames under same ip, finds outside condition disqualified. 2 users having 4 usernames registered under same ip prone fraud.

my problem is: how uniquely identify each user.

i cannot use ip, because there can multiple users behind router or worse use mobile gprs, etc.

i cannot use social identification number, cause might gather of relatives , use social number.

emails, can set email pretty easy.

i`m running out of ideas.

later edit: enforce security key, generate 1 each user subscribed, key, user can later add user, on different ip.

i know not secure, make harder

you use phone number, , require them receive registration text. still use friend's phone, lot less prone abuse.

is there cash prize involved? if so, there's nothing can prevent abuse.