I have this problem: In Fig. 22-51, two curved plastic rods, one of charge q and the other of charge q , form a circle of radius R 8.50 cm in an x-y plane. The x axis passes through both of the connecting points, and the charge is distributed uniformly on both rods. If q pC 15.0 , what are the (a) magnitude and (b) direction (relative to the positive direction of the x axis) of the electric field E produced at P, the center of the circle?

In the solution I find this:

From symmetry, we see that the net field at P is twice the field caused by the upper semi-circular charge, that is q (and that it points downward).

I don't know why P is twice the filed caused by the upper semi circular charge.

Thanks a lot if you can explain to me.