title: Sample without replacement, or duplicates, in R
body: <p>I have a long list, which contains quite a few duplicates, say for example 100,000 values, 20% of which are duplicates.  I want to randomly sample from this list, placing all values into groups, say 400 of them.  However, I don't want any of the subsequent groups to contain duplicate values within them - i.e. I want all 250 members of each group to be unique.</p>
  
  <p>I've tried using various permutation methods from vegan, picante, EcoSimR, but they don't do quite what I want, or seem to struggle with the large amount of data.</p>
  
  <p>I wondered if there was just some way of using the sample function that I can't figure out?  Any help or alternative suggestions would be much appreciated...</p>