Towards the end of the day, i'll have my contacts in and i'll start seeing double. Why does this happen?

Fatigue. We call it decompensation of a underlying phoria when weak or tired. Thus eyes don't fuse on an image well. Medications can cause it. If related to near work, could be a convergence insufficiency. Also tear film problems like dry eye can disrupt vision.
Fatigue. At the end of the day your eyes have done a lot of focusing. Sometimes, the power of the contact lenses can uncover a bit of strabismus which is normally well controlled. Also, dry eyes can cause a subtle ghost image especially when blinking. Good idea to be seen by your ophthalmologist and if necessary, adjust the focusing or treat for dry eyes.