Detect only one keypress in C++

I'm trying to use the glut framework to handle keypresses for a game I'm writing in c++. When a specific key is pressed, I want to start drawing a new curve and so I create a new curve and add it to my container of curves. However, when I use onKeyboard and debug I see that this function is called over and over when a key is pressed, not just on the initial press. Therefore, it's creating about 12 curves every time a key is pressed. My code for onKeyboard is below. Any help would be appreciated greatly!

void onKeyboard(unsigned char key,int x, int y) {
    keysPressed[key] = true;
    switch (key) {
            case 'l':
                curvesContainer.addCurve(new lCurve());
                break;
            case 'p':
                curvesContainer.addCurve(new pCurve());

    }
    glutPostRedisplay();
}

You would simply check if keyPressed[key] is already true, and only add your curve if it is false.

void onKeyboard(unsigned char key,int x, int y) 
{
    if (keysPressed[key] == false)
    {        
        keysPressed[key] = true;

        switch (key) 
        {
            case 'l':
                curvesContainer.addCurve(new lCurve());
                break;
            case 'p':
                curvesContainer.addCurve(new pCurve());
                break;
        }
    }

    glutPostRedisplay();
}

You will additionally need to clear keysPressed[key] in an onKeyUp event, using glutKeyboardUpFunc

void onKeyUp(unsigned char key,int x, int y) 
{
    keysPressed[key] == false;       
}