title: Shiny reactiveUI hangs with multiple uiOutput calls on same condition variable
body: <p>I am trying to make a reactive UI with sliders that drop in and out via a dropdown in shiny. I have a server with reactiveUI sliders (server.R):</p>
  
  <pre><code>library(shiny)
  shinyServer(function(input, output) {
    output$slider1 &lt;- reactiveUI(function() {
      sliderInput("s1", "slide 1", min = 1,  max = 100, value = 1)  
    })
  
    output$slider2 &lt;- reactiveUI(function() {
      sliderInput("s2", "slide 2", min = 1,  max = 100, value = 1)   
    }) 
  })
  </code></pre>
  
  <p>I can run the server fine with the following code (ui.R): </p>
  
  <pre><code>library(shiny)
  shinyUI(pageWithSidebar(
    headerPanel("Hello Shiny!"),
    sidebarPanel(
  
      selectInput("dataset", "number of buckets:", 
                  choices = c(1,2,3)),
  
      conditionalPanel(
        condition = "input.dataset==2",
        uiOutput("slider1"),uiOutput("slider2")),
  
      conditionalPanel(
        condition = "input.dataset==1",
        sliderInput("s1", "slide 1", min = 1,  max = 100, value = 1) 
        )
    ),
    mainPanel(
    )
  ))
  </code></pre>
  
  <p>but if I try to make both conditionalPanels call uiOutput, the server freezes: </p>
  
  <pre><code>library(shiny)
  shinyUI(pageWithSidebar(
    headerPanel("Hello Shiny!"),
    sidebarPanel(
  
      selectInput("dataset", "number of buckets:", 
                  choices = c(1,2,3)),
  
      conditionalPanel(
        condition = "input.dataset==2",
        uiOutput("slider1"),uiOutput("slider2")),
  
      conditionalPanel(
        condition = "input.dataset==1",
        uiOutput("slider1") 
        )
    ),
    mainPanel(
    )
  ))
  </code></pre>
  
  <p>I have played around with this, and found that it happens anytime that use the same condition variable and multiple uiOutput calls. Any suggestions? Thanks.</p>