Why does Dynamo DB throw an error when updating a map key with the same value?

Question

I am trying to run a simple update query, but got an error when I tried to update the key of a map to the same value. Is there a technical reason this would be disallowed? or some kind of best-practice that I am violating by trying to do this?

Error:

ValidationException: Invalid UpdateExpression: Two document paths overlap with each other; 
must remove or rewrite one of these paths; path one: [questions, What is xx?], path two: [questions, What is xx?]

Query object:

{
  TableName: 'notesTable',
  Key: { topic: 'My tooic' },
  ExpressionAttributeNames: { '#qq': 'What is xx?', '#updq': 'What is xx?' },
  ExpressionAttributeValues: { ':updans': 'new answer' },
  UpdateExpression: 'REMOVE questions.#qq SET questions.#updq = :updans'
}

Answer 1

Multiple ways to deal with scenario when same key needs to be updated. Instead of removing and updating the same key, we can simply SET the key , which replaced the value anyhow.

So, simple way is to send different updateExpression each time.

const qq = "What is xx2?";
const updq = "What is xx?";
let expressionAttributeNames;
let UpdateExpression;

if (qq === updq) {
  expressionAttributeNames = { "#updq": "What is xx?" };
  UpdateExpression = "SET questions.#updq = :updans";
} else {
  expressionAttributeNames = { "#qq": "What is xx1?", "#updq": "What is xx?" };
  UpdateExpression = "REMOVE questions.#qq SET questions.#updq = :updans";
}

docClient.update(
  {
    TableName: "test",
    Key: {
      id: "My tooic",
    },
    ExpressionAttributeNames: expressionAttributeNames,
    ExpressionAttributeValues: { ":updans": "new answer1" },
    UpdateExpression: UpdateExpression,
  },
  function (error, result) {
    console.log("error", error, "result", result);
  }
);