ASP.NET Core 7 Web API : simultaneous Post/Put call how to prevent update a data at the same time?

I'm currently doing a mock voting app using ASP.NET Core 7 Web API and Blazor WebAssembly, I still do not have database for this, so what I did is I use AddSingleton for my repository, for me to test this.

Here's our model:

public class Candidate
{
    public int Id { get; set; }
    public string? CandidateName { get; set; }
    public int VoteCount { get; set; }
}

which has this data

1 John Doe 
2 Max Smith
3 Mark Spencer

Now here's my code on doing a vote.

Here's my repository code:

public void VoteCandidate(int id)
{
    Candidate candidate = _candidates.FirstOrDefault(a => a.Id == id);

    if (candidate != null)
    {
        int currVote = candidate.VoteCount;
        candidate.VoteCount = currVote++;
    }
}

Here's my API code

[HttpPut]
public IActionResult SubmitVote(int id)
{
    _voteRepository.VoteCandidate(id);
    return Ok(_voteRepository.GetCandidateById(id));
}

We tried this and it's working fine, but when we tried to vote at the same time for Max Smith, the vote only returns 1.

Is there a way to handle a simultaneously call in API? Sorry we are all beginners when it comes to this, I hope you can help us.

Thank you.