Error: Too Many Ties in knn

Question

I am trying to make a classification in a database using the KNN, without success with the following code:

#Fase 2: KNN
#Preparação df para o KNN
dfNormal = df
dfNormal[, -length(df)] = scale(dfNormal[, -length(df)])
train_index = sample(1:nrow(dfNormal), 0.6*nrow(dfNormal), replace = FALSE)
treino = data.frame()
treino = dfNormal[train_index,]
teste = data.frame()
teste = dfNormal[-train_index,]

#Execução do KNN
library("class")
Knn_Testes = list()
acuracia = numeric()
for(k in 1:20){
  Knn_Testes[[k]] = knn(treino[, -length(treino)], teste[,-length(treino)], treino$stress, k, prob=TRUE)
  acuracia[k] = sum(Knn_Testes[[k]]==teste$stress)/length(teste$stress)*100
}

But I get the following error:

Error in knn(treino[, -length(treino)], teste[, -length(treino)], treino$stress,  : 
  too many ties in knn