Unique Random Number using ColdFusion Function RAND()

I've been playing with the RAND() function, trying to generate a unique random number so that I can use that random number as a sort of key to send the user, to confirm their email address.

I read on this forum HouseOfFusion that an easy way to create a unique random number is to create a 9 digit random number with the RAND() Function and then take today's date and today's time, format it to a number, followed by another 9 digit number using the RAND() function. Add all 3 together and you got a number that is nearly impossible to duplicate. Here is the code to make that happen.

<cfset tdstamp="#DateFormat(Now(),'mmddyyyy')##TimeFormat(Now(),'hhmmss')#">
    
    <cfset randomnum1=RAND()*100000000>
    
    <cfset randomnum2=RAND()*100000000>
    
    <cfset uniquenumber="#randomnum1##tdstamp##randomnum2#">

Although this number is nearly impossible to be recreated, is there another way to actually create a truly random number and make that number never show up again?

My first thought is to simply generate that number, insert it into the database. Then next time a unique random number is generated, query the database to see if that number has ever been created. But what if you are not storing the number anywhere, how could you generate a truly unique randomly generated number?

If anyone has any suggestions, I would love to hear them.

Thanks in advance.