Alternative Specific Conditional Logit in R

Question

I am trying to run an alternative specific conditional logit in R but keep getting the error when I use the mlogit function: "Error in dfidx::dfidx(data = data, dfa$idx, drop.index = dfa$drop.index, : the two indexes don't define unique observations"

The following code is the data I am using and the mlogit function.

dat <-read.csv("https://raw.githubusercontent.com/ryangifford/discrete_choice_experiment/master/Data/FinalData.csv")


model <- mlogit(Choice ~ Rail + Teton + Dirt + Gravel + Paved + Dist + Cost, 
                data = dat)

Any help to resolve this error would be greatly appreciated. Thanks.