PYTHON-5488 append_metadata should not add duplicates
          
            #2710
        
      | Job | Run time | 
|---|---|
| 56s | |
| 2m 34s | |
| 11m 52s | |
| 4m 3s | |
| 4m 29s | |
| 11m 11s | |
| 3m 54s | |
| 2m 34s | |
| 11m 21s | |
| 8s | |
| 53m 2s | 
append_metadata should not add duplicates
          
            #2710
        
| Job | Run time | 
|---|---|
| 56s | |
| 2m 34s | |
| 11m 52s | |
| 4m 3s | |
| 4m 29s | |
| 11m 11s | |
| 3m 54s | |
| 2m 34s | |
| 11m 21s | |
| 8s | |
| 53m 2s |