Identitifers can be rebound in both F# compiled code and interactive code. It's different to mutation because although you give a new meaning to that name code from before still sees the old meaning. I like to think of it as creating a new binding that just happens to have the same name as an old one. Consinder the following code snippet:

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#light
let main() =
    let x = "hello World"
    let printx() =
        printfn "%s" x
    printx()
    let x = 1
    printx()

When executed main will print "hello world" to the console twice. The fact we've later given a different meaning to "x" has no effect on this.

The easiest way to flush the interactive console is to use the #quit command. The process thats hosting that particular session will close down and visual studio will automatically start a new one for you, ensuring everything is nice and fresh.