Played it again recently and found this:
As you already know, all weapons including Laser Pistol and Circular Saw are taken from Alex as he arrives to Underground Lair. The thing is that the script that takes away all weapons seems to work like "remove 1 (!) weapon from slot #, go to next slot, repeat". Now here comes the good part: you can actually collect Laser Pistol twice, the second time being when you return to Shepherd's house after Hell Descent. And it will stay when every other weapon is removed from inventory, which will make the fight against Curtis on Hard mode much easier (as well as subsequent fight with rifle-wielding Order soldier).
It may be a PC-only bug, so PS3 and Xbox 360 users please report.
