Add warning for duplicate times in OutputVar in `read_var`

In https://github.com/CliMA/ClimaDiagnostics.jl/issues/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.

Start by locating the read_var entry point and how OutputVar stores or checks time values. Reproduce the duplicate-date case described in issue 153, then add a warning when duplicate times are present and verify that the warning appears without deciding whether values should be removed.

Written by the indexing model from the issue text.