To save the data you can use static-data if it is not large. Probably best you do not save the whole array only some kind of ID for each. That makes it then also easier to compare later to filter out the ones you did already process.
Here some information about static-data:
Here also some other ways to store data:
For the comparison, you can then use a Function-Node. With it, you can then use for example a JavaScript filter function on the items or reference the data in an IF-Node and filter there. Not sure if you have JavaScript experience. If yes it should not be too hard, if you do not have, then it is probably harder. But here an example:
Apart from that should you find multiple examples if you search here in the forum for “filter”.