apollo-link is called every time when "setLink" method is called

[Moniv9]

Intended outcome:
Trying to set new link after apollo client initialisation by using the public method setLink. With this method, any new link
should be invoked only once.

Actual outcome:
By using the above method, can see new apollo-link is being called every time setLink is called/set.

How to reproduce the issue:
Here is the code to reproduce it.

Test component

import * as React from "react";
import gql from "graphql-tag";
import { ApolloLink, useApolloClient } from "@apollo/client";

export const Test = () => {
  const OtherLink = new ApolloLink((operation, forward) => {
    console.log("custom link is called");

    return forward(operation);
  });

  const client = useApolloClient();

  /* by calling it twice, we will observe that "custom link is called" is called twice.
     Ideally it should be called only once */
  client.setLink(ApolloLink.from([OtherLink, client.link]));
  client.setLink(ApolloLink.from([OtherLink, client.link]));

  client
    .query({
      query: gql`
        {
          widgets
        }
      `
    })
    .then((data) => {
      //console.log(data);
    });

  return <>Test</>;
};

Parent component

import "./styles.css";
import {
  ApolloClient,
  ApolloProvider,
  ApolloLink,
  InMemoryCache,
  Observable
} from "@apollo/client";
import { Test } from "./Test";

export default function App() {
  const link = new ApolloLink((operation) => {
    return new Observable((observer) => {
      observer.next({
        data: {
          widgets: [{ name: "Widget 1" }, { name: "Widget 2" }]
        }
      });
      observer.complete();
    });
  });

  const apolloClient = new ApolloClient({
    cache: new InMemoryCache(),
    link: ApolloLink.from([link])
  });

  return (
    <ApolloProvider client={apolloClient}>
      <div className="App">
        <h1>Hello CodeSandbox</h1>
        <Test />
      </div>
    </ApolloProvider>
  );
}

Codesandbox

https://codesandbox.io/s/wizardly-tu-94i5f?file=/src/App.js

Versions

  System:
    OS: macOS 11.2.3
  Binaries:
    Node: 12.13.1 - ~/.nvm/versions/node/v12.13.1/bin/node
    npm: 6.14.11 - ~/.nvm/versions/node/v12.13.1/bin/npm
  Browsers:
    Chrome: 90.0.4430.93
    Firefox: 83.0
    Safari: 14.0.3
  npmPackages:
    @apollo/client: 3.3.15 => 3.3.15 
    @apollo/react-common: ^3.1.4 => 3.1.4 
    @apollo/react-hooks: 4.0.0 => 4.0.0 
    apollo-link-timeout: 4.0.0 => 4.0.0 

[hwillson]

@Moniv9 for now this is something you'll need to manage in your code. setLink will add whatever link you pass it to the link chain; it won't try to deduplicate links. If you think it should, please open a feature request in http://github.com/apollographql/apollo-feature-requests/issues. Thanks!