REF: deduplicate group min/max
ASVs look unaffected:
<details>

```
before           after         ratio
     [05a0e98a]       [16d06e22]
     <master>         <ref/deduplicate_grp_min_max>
         33.0±3ms         33.3±1ms     1.01  gil.ParallelGroupbyMethods.time_loop(2, 'max')
       32.2±0.4ms         34.5±3ms     1.07  gil.ParallelGroupbyMethods.time_loop(2, 'min')
         64.1±1ms       65.6±0.5ms     1.02  gil.ParallelGroupbyMethods.time_loop(4, 'max')
         65.3±3ms         65.5±1ms     1.00  gil.ParallelGroupbyMethods.time_loop(4, 'min')
         20.2±2ms       20.2±0.8ms     1.00  gil.ParallelGroupbyMethods.time_parallel(2, 'max')
         21.3±1ms       20.1±0.8ms     0.95  gil.ParallelGroupbyMethods.time_parallel(2, 'min')
       25.1±0.7ms       25.3±0.5ms     1.01  gil.ParallelGroupbyMethods.time_parallel(4, 'max')
       25.0±0.4ms         26.5±2ms     1.06  gil.ParallelGroupbyMethods.time_parallel(4, 'min')
         125±50ms          104±1ms    ~0.83  groupby.GroupByCythonAgg.time_frame_agg('float64', 'max')
          105±2ms          107±2ms     1.02  groupby.GroupByCythonAgg.time_frame_agg('float64', 'min')
```
</details>

LGTM thanks @mzeitlin11 