Will 75mg of amitriptyline cause an overdose, i took 50mg and another 25mg half hour later

No! Not usually: This is within normal therapeutic range. You should not ever take other peoples medications. Strictly follow your doctor's advice about what to take and how much. The decision making process in medicine is exceedingly complex. Laypeople sometimes learn a little and then think they know enough to make medical decisions. This is dangerous. As we say, "only a fool has himself as his doctor".

No: Many people take much higher doses of Amitriptyline on a daily basis. The highest daily dose i've ever prescribed is 300 mg. Nevertheless, please be careful to take your medication as your doctor prescribes it. You may notice some unpleasant effects resulting from taking too much, like increased constipation, dry mouth, blurry vision, difficulty urinating and sedation. These should all pass.