How to remove dublicate words in a file?

I have a text file that contains:
alt="black" alt="white" alt="red" alt="green" alt="black"
alt="red" alt="white"...

how to make output like this:
alt="black" alt="white" alt="red" alt="green" ???

Use queue of strings:
try word by word - if it is in queue do nothing, if not enqueue it.
At the end, dequeue all strings.