Ignore "Content already exists in this project" error

### Please avoid duplicates

- [X] I checked [all open bugs](https://github.com/gr2m/github-project/issues?q=is%3Aissue+is%3Aopen+label%3Abug) and none of them matched my problem.

### Reproducible test case

_No response_

### Please select the environment(s) that are relevant to your bug report

- [ ] Browsers
- [ ] Node
- [ ] Deno

### Versions

latest

### What happened?

I haven't tried to reproduce it, but I got this error in a production app:

```
GraphqlResponseError: Request failed due to following response errors:
- Content already exists in this project
```

I assume it occurs when trying to add an issue or pull requse to a project that was already added, but I thought that theGraphQL API is already handling that gracefully. Maybe there is some racing condition?

### Would you be interested in contributing a fix?

- [ ] yes

Start by reproducing the reported GraphQLResponseError when adding an issue or pull request already present in a project, using the latest version and the relevant Node or Deno environment. Define done as graceful handling without the “Content already exists in this project” error; the report provides no test case or file to start from.

Written by the indexing model from the issue text.