Concatenating char[] overwrites data

I have this code:

  char tempString[10];
  char humString[10];
  char dataString[20];


void loop() {
  //For DHT22 Grove Pro
  float h = dht.readHumidity();
  float t = dht.readTemperature();
  dtostrf(t, 4, 2, tempString);  //convert flat to char
  dtostrf(h, 4, 2, humString);
  Serial.println(tempString);
  Serial.println(humString);
  strcpy(dataString,tempString);
  strcpy(dataString,humString);
  Serial.println(dataString);
}

But the serial monitor puts this out:

29.50
69.30
69.30

So instead of concatenating it, it is overwriting it. Why?

The strcpy() function is to copy strings, not concatenate. The strcat() function for concatenate.

My friend, strcpy is not strcat.