data validation

I was trying to validate a form with the following code:

if (count(Address[Address_Name == input.Address_Name && client_code == input.client_code]) > 0)
{
alert ("An address with the name already exists for this client. Please enter a new name.");
cancel submit;
}

But it is not working. Sometime the validation works and throws the alert and sometimes the user can enter the data successfully. I thought it was a really easy validation.

Am I doing something wrong or there is something wrong with my application? The only possible problem I can see in this code is:

Address[Address_Name == input.Address_Name && client_code == input.client_code]

Can I do something like this? Or I will have to do this:

Address[client_code == input.client_code]

and fetch all the records and inside a loop check for the address name?