Clerks are entering duplicate Legistar MatterSponsors

### Describe the Bug

_A clear and concise description of the bug._

```Python
import requests
resp = requests.get("http://webapi.legistar.com/v1/kingcounty/Matters/22552/Sponsors")
print(resp.status_code)
for sponsor in sponsors:
print(sponsor["MatterSponsorName"])

# Joe McDermott
# Jeanne Kohl-Welles
# Jeanne Kohl-Welles
# Joe McDermott
```

### Expected Behavior

_What did you expect to happen instead?_

```Python
import requests
resp = requests.get("http://webapi.legistar.com/v1/kingcounty/Matters/22552/Sponsors")
print(resp.status_code)
for sponsor in sponsors:
print(sponsor["MatterSponsorName"])

# Joe McDermott
# Jeanne Kohl-Welles
```

### Reproduction

_Steps to reproduce the behavior and/or a minimal example that exhibits the behavior._

Given above

### Possible solutions

Remove duplicates after querying `MatterSponsors` from Legistar API.