Add warning for duplicate times in OutputVar in `read_var`

[ph-kev]

In CliMA/ClimaDiagnostics.jl#153, NetCDF files produced from multiple simulations with restarts can share the same dates. For now, it would be helpful to throw a warning that duplicate time values are in the OutputVar.

I am not sure if removing the time value should be something that ClimaAnalysis handle or the user should handle.

[ph-kev]

I think ClimaAnalysis should provide a function that remove duplicates values in the time dimension or more broadly, any dimension that the user want. It might matter which time value to keep or not.