How many items in a list can a human create without repetition? Consider the question "Name as many animals as you can."

An average person could no doubt list off about 20-30 animals without repetition.

This is odd seeing as we have a 7-9 item maximum short term memory.

The theory would suggest that as soon as we got to animal 10 we would start repeating the same animals without realising it. Whereas of this is plainly not what a person does. We sort of know "I've already said that one". Even though we wouldn't be able to repeat the whole list back.

Is there a limit to this kind of memory? How many animals can a human list off until they forget whether or not they have named a certain animal? Is it time based or number based? 

It feels like we're marking off different neurons in the brain when we say them. So we don't say them again.
Recalling items from short term memory is a very different task from creating a list of all unique items. There are a number of strategies for recalling items from short term memory without repeating, you can remember all the items in the list in a random order and all the items you have recalled or you can remember all the items in a fixed order and where you are in the list. When creating a list of unique items you need to create a rule, remember this rule and the number of items you have recalled. The list itself is in long term memory

For example, if you are name as many numbers as you can without repetition, you could say the positive integers 1, 2, 3, ..., N. Equally valid would be the even positive integers 2, 4, 6, 2N. You could also list the Fibonacci sequence 1, 1, 2, 3, 5, ..., $\frac{\phi^n-(-\frac{1}{\phi})^n}{\sqrt{5}}$ or if you know them the digits of $\pi$ 3, 1, 4, 1, .... In all of these cases there is a limit to how far you can get since you need to remember the number of items you have given.

Numbers are obviously easier to create rules for than animals, but for animals you could do something like ant, bat, cow, dog, eel, fly, ..., zho, anoa, bear, cuis, deer, ..., zobo, addax, bison, camel, dingo, ..., zebra, ...  I am not sure how far one can really go with this rule, but it will get you pretty far