The timeline's left, diamond and right buttons navigate to the next or previous keyframe. When the diamond is lit up, the CTI (current time indicator) is on a keyframe. If you click the diamond when it is open, you will set a new keyframe at the current position of the CTI.
Issue is if you chage the properties of a layer [scale, position, opacity and rotation ect.] you will get a new keyframe. In that case keyframe at the left is duplicated.