title: Ambiguos "Error: NEXUS__UNKNOWN__TYPE was already defined and imported as a type" error in nexus graphql
body: <p>I'm getting the following error when using nexus to define a graphql schema with apollo-server.</p>
  <p><code>Error: NEXUS__UNKNOWN__TYPE was already defined and imported as a type</code></p>
  <p>The stacktrace doesn't give much information as to where the issue is occurring or to what the problem is. The project has 20+ models and dozens of resolvers so it's quite hard to debug.</p>
  <pre><code>Error: NEXUS__UNKNOWN__TYPE was already defined and imported as a type, check the docs for extending types
      at extendError (/Users/username/Documents/folder/folder/graphq-nexus-prisma-api/node_modules/nexus/src/builder.ts:1744:2)
      at SchemaBuilder.addType (/Users/username/Documents/folder/folder/graphq-nexus-prisma-api/node_modules/nexus/src/builder.ts:603:8)
      at SchemaBuilder.missingType (/Users/username/Documents/folder/folder/graphq-nexus-prisma-api/node_modules/nexus/src/builder.ts:1212:5)
      at SchemaBuilder.getOrBuildType (/Users/username/Documents/folder/folder/graphq-nexus-prisma-api/node_modules/nexus/src/builder.ts:1540:4)
      at SchemaBuilder.getOutputType (/Users/username/Documents/folder/folder/graphq-nexus-prisma-api/node_modules/nexus/src/builder.ts:1471:10)
      at SchemaBuilder.buildOutputField (/Users/username/Documents/folder/folder/graphq-nexus-prisma-api/node_modules/nexus/src/builder.ts:1349:52)
      at /Users/username/Documents/folder/folder/graphq-nexus-prisma-api/node_modules/nexus/src/builder.ts:1307:7
      at Array.forEach (&lt;anonymous&gt;)
      at SchemaBuilder.buildOutputFields (/Users/username/Documents/folder/folder/graphq-nexus-prisma-api/node_modules/nexus/src/builder.ts:1306:7)
      at fields (/Users/username/Documents/folder/folder/graphq-nexus-prisma-api/node_modules/nexus/src/builder.ts:1009:33)
  </code></pre>
  <p>Any help appreciated.</p>