It's not all ways a good idea to cast the same spell twice. Period.
Matter of fact I would recommend you double casting as little as possible.
Most of the time if a spell's fades there is a reason for it that can either be fixed or learned from and then applied to future situations.
If you cast a money spell, you get money, it's good, then all of a sudden you have a bit less money, then less, and less, and less...it's not necessarily time to cast the same spell again. Unless something happened during the casting process that dictated such a thing to occur it most likely has to do with financial business and not the spell's effectiveness itself. I can tell you that a spell's effectiveness can basically last forever but rarely will. It's like the energy you use forgot what it was doing it's because times change and so do the conditions spells are cast under. In other words if you lose your job or you're getting fewer hours at work, don't expect that money to be coming out of nowhere. You have to work with the spell and not expect it to do everything. Conditions have to be right, and the purpose has to be possible.
Otherwise every single witch would be immortal, rich, stay-at-home hobbyists.
Another thing is if you doubt or question the spell the mixed signals you're sending out can actually dispute the spell at work and sometimes counteract the effect.