How to return a message warning that registration already exists?

I have a method in CepController RegisterCep that before doing the registration makes some validations and between them, check if there is already a registration for that Cep. I would like to implement a warning to inform the user about the existence of the record without giving a Post on the page.

Controller method:

public IActionResult CadastrarCep(CepViewModel cep)
        {
            using (var sqlConnection = new SqlConnection(_config.GetConnectionString("DBConect")))
            {
                if (!ModelState.IsValid)
                {
                    return View("Cadastro", cep);
                }

                if (cep.TakeCepById(cep.CepCodigo, _config) != null)
                {
                    ModelState.AddModelError("Cep", "Cep já existente.");
                  //  return View("Cadastro", cep);

                }
                cep.CepSetor = 0;
                cep.CepRegiao = "";
                cep.CepRota = "";

                StringBuilder strSql = new StringBuilder ("INSERT INTO Cep (CepCodigo, CepLogr, CepEnd, CepCompl, CepBairro, CepCidade, CepUF, CepSetor, CepRegiao, CepRota) ");
                strSql.Append(" VALUES( @CepCodigo, @CepLogr, @CepEnd, @CepCompl, @CepBairro, @CepCidade, @CepUF, @CepSetor, @CepRegiao, @CepRota)");
                sqlConnection.Execute(strSql.ToString(), cep);


                return View("Cadastro", new CepViewModel());

            }
        }

View:

@model WebApplication1.Models.CepViewModel
<h2>Manutenção de Cep</h2>
<div class="container" style="margin-top: 50px;">
    <form asp-controller="Cep" asp-action="Cadastro" method="post">
        <div class="form-group">
            <label class="control-label">CEP</label>
            <input asp-for="CepCodigo" maxlength="8" min="8" class="form-control" type="text" />
            <span asp-validation-for="CepCodigo"  class="text-danger"></span>
        </div>

        <div class="form-group">
            <label class="control-label">LOGRADOURO</label>
            <input asp-for="CepLogr"  class="form-control" type="text"  value="@Model.CepLogr" />
        </div>
        <div class="form-group">
            <label class="control-label">ENDEREÇO</label>
            <input asp-for="CepEnd"  class="form-control" type="text"  value="@Model.CepEnd" />
        </div>
        <div class="form-group">
            <label class="control-label">COMPLEMENTO</label>
            <input asp-for="CepCompl"  class="form-control" type="text"  value="@Model.CepCompl" />
        </div>
        <div class="form-group">
            <label class="control-label">BAIRRO</label>
            <input asp-for="CepBairro"  class="form-control" type="text"  value="@Model.CepBairro" />
        </div>
        <div class="form-group">
            <label class="control-label">CIDADE</label>
            <input asp-for="CepCidade"  class="form-control" type="text"  value="@Model.CepCidade" />
        </div>
        <div class="form-group">
            <label class="control-label">UF</label>
            <input asp-for="CepUF"  class="form-control" type="text"  value="@Model.CepUF" />
        </div>
        <div class="form-group">
            <input type="submit" class="btn btn-primary" value="Consultar" asp-action="ConsultarCep" />
            <input type="submit" class="btn btn-default" value="Cadastrar" asp-action="CadastrarCep" />
            <input type="submit" class="btn btn-danger" value="Excluir" asp-action="ExcluirCep" />
            <input type="submit" class="btn btn-success" value="Alterar" asp-action="AlterarCep" />

        </div>
</div>
</div>
</form>