Duplicate questions added to list

[iamleeg]

Summary: As mentioned in the book, there's a bug in the way questions are added to a topic. The same question can appear in a topic's list of questions multiple times, pushing older questions off the list.

Steps to reproduce:

1. Launch app.
2. Go to the 'iPhone' topic.
3. Wait for questions to load.
4. Go 'back' to the topic list.
5. Go to the 'iPhone' topic again.

Expected results:

6a. Any new questions in this topic are fetched and added to the list, and remaining spaces are occupied by existing questions.

Actual results:

6b. The newest 25 questions in this topic are fetched and added to the list, even if some of them are already present. For those that were present, there are now two copies of the question in the list.

Regression:

Not applicable, as this is the first release.

Notes:

Readers might like to fork the project and fix this issue using a test-driven approach.