WD-40 won't make the electrical connection better. Either a wire has fallen off, or the switch has gone bad. When you pull the arm down, once it reaches the bottom it should hit the microswitch that Ken from Ohio Gaming posted a couple of nice pictures of in
Reply #4. You will need to ensure that
both wires are connected (you said you only felt one -- that's likely your issue) and that the arm is still activating the switch when pulled all the way down. I'd suggest unplugging the machine, remove the hopper, and then see if you can stick your head in there enough to see the bottom of the mechanism to look for the missing/unhooked wire.
The only way that the machine should give you two credits per coin inserted is if you're using a chip that supports tokenization ("Montana credits"), and you have that enabled. If your machine has always behaved that way, you'll just need to change some settings in the service menu. We'll need to start by you telling us what SP (program) chip is in your machine. You can determine that with the instructions found here:
http://www.newlifegames.net/spset/SPSS.htmIf this is recent behavior, would you be able to post a video of that issue happening? It would be extremely unusual for a machine with a typical SP chip to award two credits per coin inserted -- the casinos would have gone broke if that happened with any frequency.