state approach using

JetpackState.kt

@Composable
fun AddTask() {
   val title = remember { mutableStateOf("") }
   val titleExist = remember { mutableStateOf(false) }
  
  if (titleExist.value) Text(
     modifier = Modifier.padding(5.dp),
     text = "Title already exist",
     style = TextStyle(color = Color(0xFFF50B0B)) + MaterialTheme.typography.body1
  )
  
   TextField(
      value = title.value,
      onValueChange = { item ->
             title.value = item
             titleExist.value = isTitleExist(title)
      },
      label = {
             Text(
                  modifier = Modifier.padding(5.dp),
                  text = "Title of Task",
                  style = TextStyle(color = Color(0x77666666)) + MaterialTheme.typography.h6
             )
       },
         modifier = Modifier.fillMaxWidth(),
         textStyle = MaterialTheme.typography.h6,
         backgroundColor = Color.White
        )
}

fun isTitleExist(title: MutableState<TextFieldValue>): Boolean {
    val item = AppMain.taskList.find { it.title == title.value.text.trim() }
    return item != null
}