Ao rodar a aplicação ela apresentou esse erro. o que poderia estar ocorrendo?

An unhandled exception occurred while processing the request.
InvalidOperationException: Sequence contains more than one element
System.Linq.Enumerable.SingleOrDefault<TSource>(IEnumerable<TSource> source)

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InvalidOperationException: Sequence contains more than one element
System.Linq.Enumerable.SingleOrDefault<TSource>(IEnumerable<TSource> source)
lambda_method(Closure , QueryContext )
Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler+<>c__DisplayClass17_1.<CompileQueryCore>b__0(QueryContext qc)
System.Linq.Queryable.SingleOrDefault<TSource>(IQueryable<TSource> source)
CasaDoCodigo.Repositories.PedidoRepository.AddItem(string codigo) in PedidoRepository.cs
+
            var produto = contexto.Set<Produto>().Where(p => p.Codigo == codigo).SingleOrDefault();
CasaDoCodigo.Controllers.PedidoController.Carrinho(string codigo) in PedidoController.cs
+
                pedidoRepository.AddItem(codigo);