Assigning Variables In A Formula

I think I'm using the correct terminology with the term "variable", but to explain what I'm trying to do, I want to get RAND() to hold the first returned variable so I can compare it to other places RAND() has been used & if the returned value is the same as the other place, then run RAND() again.

Here is my basic formula (where $B$2 is 50):

=IF(ROUND(RAND()*$B$2, 0)+1=A4, ROUND(RAND()*$B$2, 0)+1, ROUND(RAND()*$B$2, 0)+1)

I'm trying to say in the formula above that if rand = what was already in A4 then run random again -- but this doesn't keep if from returning the same value as A4 on the second pass.

What would work is something like (where X is the assigned variable):

=IF(X=ROUND(RAND()*$B$2, 0)+1=A4, ROUND(RAND()*$B$2, 0)+1, X)

But it doesn't appear you can use X in a formula (only in a macro which I don't want to use) --