z + z = 2z. z is the same as saying 1 * z. you can combine like terms such as x+2x that equals 3x.
Anytime you have numbers and variables together such as 2x, 5x, 5z, 3y, it means to take the number and multiply it times the value of the unknown (which is what you will figure out later as you progress in algebra)
So like terms are z+z, x+2x, y+3y, 5p-3p, etc.
So the answer to your question is z+z = 2z
Let me know if you have any other questions about this.