How to append a new entry in a list of 100,000 names without iterating the list each time?

I have a list of 100,000 + names and I need to append to that list another 100,000 names. Each name must be unique. Currently I iterate through the entire list to be sure the name does not exist. As you can imagine this is very slow. I am new to Java and I am maintaining a 15+ year old product. Is there a better way to check for an existing name?

We are using a Java list because that is how the original developers coded it. I don't think they planned for that many entry's. I know I need to re factor this which is why I am asking for opinions on how to make this more efficient. Currently we don't use a database for anything in the product so I would like to stay away from that if possible.
Ok - but it still raises the question in my mind as to how that data is being used.
I gave you a couple of options that will take care of the UNIQUE requirement (HashTable, HashMap) but the BEST solution depends on:
1. How often new entries are made
2. How often entries are deleted
3. How often entries are changed
4. How often entries are accessed
5. How the data is actually used
If you just have a one time requirement to merge the two lists then just do it and get it over with - it won't really matter how you do it.
But Hash classes will present their own performance issues if the typical access is for many, most or all of that 200k+ entries.
Without knowing the full set of requirements we can't really know just what part of the process needs to be optimized.

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How to append a new entry in a list of 100,000 names without iterating the list each time?

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