nasty error message when index sets contain repeated elements

[mlubin]

julia> m = Model();

julia> s = [:x,:x,:y];

julia> @defVar(m, x[s])
Error showing value of type JuMPDict##38282{Variable}:
ERROR: access to undefined reference
 in dictstring at /home/mlubin/.julia/v0.3/JuMP/src/print.jl:192
 in print at /home/mlubin/.julia/v0.3/JuMP/src/print.jl:480
 in show at /home/mlubin/.julia/v0.3/JuMP/src/print.jl:477
 in anonymous at show.jl:1067
 in with_output_limit at ./show.jl:1044
 in showlimited at show.jl:1066
 in writemime at replutil.jl:2
 in display at REPL.jl:117
 in display at REPL.jl:120
 in display at multimedia.jl:149
 in print_response at REPL.jl:139
 in print_response at REPL.jl:124
 in anonymous at REPL.jl:541
 in run_interface at ./LineEdit.jl:1349
 in run_frontend at ./REPL.jl:763
 in run_repl at ./REPL.jl:169
 in _start at ./client.jl:398

[mlubin]

This could also fall out of #192.

[joehuchette]

What should we even do in this situation? Create two variables, only one of which is accessible?

[mlubin]

I'd say we should throw an error.