Keypress reading

I wrote this simple code to catch users key presses and write them to a text file.

Code:

.......... .......... void main() { ofstream ofile; ofile.open("keys.txt"); int keyConst=65; //reg(); while(true){ keyConst++; if(keyConst==91) keyConst=65; if(GetAsyncKeyState(keyConst)){ switch(keyConst){ case 65: ofile.put('a'); break; case 66: ofile.put('b'); break; case 67: ofile.put('c'); break; case 68: ofile.put('d'); break; case 69: ofile.put('e'); break; case 70: ofile.put('f'); break; case 71: ofile.put('g'); break; case 72: ofile.put('h'); break; case 73: ofile.put('i'); break; case 74: ofile.put('j'); break; case 75: ofile.put('k'); break; case 76: ofile.put('l'); break; case 77: ofile.put('m'); break; case 78: ofile.put('n'); break; case 79: ofile.put('o'); break; case 80: ofile.put('p'); break; case 81: ofile.put('q'); break; case 82: ofile.put('r'); break; case 83: ofile.put('s'); break; case 84: ofile.put('t'); break; case 85: ofile.put('u'); break; case 86: ofile.put('v'); break; case 87: ofile.put('w'); break; case 88: ofile.put('x'); break; case 89: ofile.put('y'); break; case 90: ofile.put('z'); break; case 48: ofile.put('0'); break; case 49: ofile.put('1'); break; case 50: ofile.put('2'); break; case 51: ofile.put('3'); break; case 52: ofile.put('4'); break; case 53: ofile.put('5'); break; case 54: ofile.put('6'); break; case 55: ofile.put('7'); break; case 56: ofile.put('8'); break; case 57: ofile.put('9'); break; } } } }

The problem is that, when I execute this program and press a key, it gets printed on the file more than once (hundreds of times). How can I prevent this from happening?
Thanks for reading.