Check for duplicate levels in isentropic_interpolation_as_dataset

Fixes #3309 by provided the user with a warning when interpolation to
a duplicate isentropic surface is requested.  This is likely a mistake
on the part of the user, but by issuing a warning rather than an
error, computations can proceed if the duplication was really
intended.

The code used here is an adaptation of the duplicate-level check found
in _parcel_profile_helper elsewhere in thermo.py.

src/metpy/calc/thermo.py

@@ -2826,6 +2826,13 @@ def isentropic_interpolation_as_dataset(
     # Ensure matching coordinates by broadcasting
     all_args = xr.broadcast(temperature, *args)
 
+    # Check for duplicate isentropic levels, which can be problematic (Issue #3309)
+    unique, counts = np.unique(levels.m, return_counts=True)
+    if np.any(counts > 1):
+        _warnings.warn(f'Duplicate level(s) {unique[counts > 1]} provided. '
+                       'The output Dataset includes duplicate levels as a result. '
+                       'This may cause xarray to crash when working with this Dataset!')
+
     # Obtain result as list of Quantities
     ret = isentropic_interpolation(
         levels,