Cannot set base providers error after Angular v9 upgrade using 'ng test'

I've got all of my unit tests updated and passing after an upgrade from v8 to v9. However, now I'm getting the following error:

Cannot set base providers because it has already been called

Research suggested that I had BrowserDynamicTestingModule or platformBrowserDynamicTesting() called twice in my application. I searched my whole app and only found them once in my test.ts file:

// This file is required by karma.conf.js and loads recursively all the .spec and framework files

import 'zone.js/dist/zone-testing';
import { getTestBed } from '@angular/core/testing';
import { BrowserDynamicTestingModule, platformBrowserDynamicTesting } from '@angular/platform-browser-dynamic/testing';

declare const require: {
  context(
    path: string,
    deep?: boolean,
    filter?: RegExp
  ): {
    keys(): string[];
    <T>(id: string): T;
  };
};

// First, initialize the Angular testing environment.
getTestBed().initTestEnvironment(BrowserDynamicTestingModule, platformBrowserDynamicTesting());
// Then we find all the tests.
const context = require.context('./', true, /\.spec\.ts$/);
// And load the modules.
context.keys().map(context);

I'm not sure where else the error could come from.