For this current mirror circuit, what is the effect of doubling of the emitter area of transistor Q3?
Doubling the emitter area of Q3 will drop that Vbe by 18 millivolts, reducing the Vce of Q1 by 18 millivolts.

Without knowing the Early Voltage, and the plot of how beta varies with Ic, I think this (doubled emitter area) is a process-specific and Imirror-specific tweak.