Answer by ekolima for Update updateselectInput() based on values of another updateSelectInput()

Adding an observeEvent for col would update col2 every time a change has been made on the selected values of col

observeEvent(input$col, {    ex_names <- colnames(iris)[-1]    ex_names <- ex_names[!ex_names %in% input$col]    updateSelectInput(session, "col2", label="Select reference countries", choices=ex_names)    })