There is a cap listed in the game files that makes it so you don't get a weapon mod in card drops after having 2 of that mod. This is normally a good thing, since it means you have a larger variety of weapon mods you can apply.

However, this changes with Crime Spree. When a Crime Spree is cashed out, all of the rewards are calculated at once. If you have only one or two weapon mods missing, you will get several thousand of that one weapon mod (depending on rewards) instead of just 2. This mod removes the limit (well okay, it doesn't remove it but I doubt any legit player is going to get more than 200 million of one weapon mod) so that when you cash out a Crime Spree the loot drops are better spread out.