The number one way to kill a spell is to *overthink* it. You have to do your work, expend your energy, do your homework, and then LET IT GO. Let the Universe do the work, without you fretting and chiseling away at your projected intention. Have faith in your work and have faith in answered prayers. Plus, magic needs time and space to manifest, kind of like a watched pot never boils.
Here is a fantastic article written by one of my colleagues on the subject. She articluates everything and more that i would say, so i recommend giving it a read:
So whenver someone tells me that they've done a bunch of spells or have hired a bunch of workers to all work on a specific target person or specific situation, it sends out a red flag to me that they are perhaps over-thinking their spellwork. Sometimes people get borderline obsessive. I will always turn down prospective clients when they've told me they've seen two or more workers all within a short window of time for the same set of circumstances (now, this is mainly only true if they've hired reputable workers, not those frauds out there who do nothing magical and just scam you for money. But i digress...).
Assuming that you're not obsessing (and please don't take offense) and assuming that you're not over-thinking your work, then i usually don't think that a couple spells *of the same intention* and for *the same future outcome* will somehow counteract or cancel eachother out.
Obviously if you do a reconciliation spell one week on your man, he doesn't come back, and then the next week you throw a curse at him, well, then yes, they will most likely mix about and muck everything up. But if you've done a honey jar spell to sweeten you ex to you and then done a contact me spell to get him thinking about you and then wear a love-drawing mojo bag on your person, then that's an example of several types of spell workings that are complimentary to each other.
If you wish to elaborate, then maybe i can give more tailored advice or opinions.