F# System.Random duplicate values

I'm trying to use System.Random in F# but keep getting duplicate values. I'm aware that if I create a new instance of System.Random each time I do .Next i might get duplicate values. What I'm trying to achive is setting a random id on each record when creating them. Here is an example of the problem in code:

[<AutoOpen>]
module Domain = 
    type Test = {id: int; ....} 

module Test =

    let r = System.Random() 

    let t create =
        {id = r.Next(); ....}


let a = Test.create
let b = Test.create

In the code above bot a and b gets the same id.

What I'm guessing is happening is that let r = System.Random() doesn't get treated as a variable but as a function which returns a new instance of Random. Is this correct? And how can I Solve this problem?

Thanks!