To duplicate any item as many times as you want do the following:
This tip allows you to jump through the various missions:
On mission one, just before you rescue the first captive (at 8 Sasgom Boulevard, Gamma Zone) save the game. Now as soon as you have rescued him and moved onto mission two, reload your saved game and rescue the captive again. This time rather than going to mission two, you'll go to mission three. If you reload your original saved game and do it again, you'll go onto mission four. You can keep this up until you get to the mission you actually want to play.
Also, if you hang onto the Delta Crystal after completing the first mission, you can use it to find the other captives on all the other missions without having to hunt around for clues.
Have two credit cards handy and if you find something you can't afford, give the man your credit card anyway and he should say something like: "That's not enough. You got another?". Give him your other credit card and he will give you the item free if you have over 50 credits on the second credit card.