Two thoughts - 1. You can easily undupe a python list like this.>>> x
[4, 4, 1, 2, 2, 3]
>>> list(set(x))
[1, 2, 3, 4]
>>>
2. I know this is the Python forum -- but want to note you could also do this task with no code using a unique fields iterator in model builder connected to the Make Feature Layer tool. The expression would include the model variable Value output by the iterator at each unique value of your field. You could cap it off using the Collect Values tool piped into AddLayerToGroup to do that final step Jake suggests. However, my experience with model builder is it is usually in my interest not to "gild the lily" so to speak, and focus on setting it up to automate tasks - not create exceedingly complex tools. Though every version adds a little more oomph!