Special characters break the allowDuplicate: false check

[jasonwilliams]

Raised from: jasonwilliams/anki#13

We have some cards which have some syntax in their front.
For example ## What is >>> in JavaScript?

Before passing this to Anki (via addNotes) we convert the characters into html safe equivalents.
So What is <code>&gt;&gt;&gt;</code> in JavaScript?

For some reason this allows duplicates in our deck.
If i remove the 3 > i cannot add more than 1 card.

How does the duplicate check work? And is there a reason it fails when certain characters are added?