An electrolyte concentration cell consists of two identical...

For an electrolyte concentration cell, the electrodes and the electrolyte are identical in both half-cells.

The standard cell potential $E^{\circ}_{\text{cell}}$ is the potential of the cell when all species are at their standard states (unit activity or $1\text{ M}$ concentration).

Since both half-cells are identical under standard conditions, their standard reduction potentials are equal.

$E^{\circ}_{\text{cell}} = E^{\circ}_{\text{cathode}} - E^{\circ}_{\text{anode}} = 0$

The actual cell potential $E_{\text{cell}}$ depends on the concentration ratio according to the Nernst equation, but the standard cell potential is always zero.

Answer: It is zero because both half-cells have the same standard reduction potential.