For the system shown, intuitively we can cut the beam at halfway between the support points G & H, so the left half is a mirror image of the right half regarding geometry, loading, support reactions, and member internal forces, so we can analyze half of the structure to obtain the same results. However, the trick is what type of support needs to be assigned to the joint at the cut.
From the full structure, it is expected that there is a positive (smiley) moment at the mid-point of the interior span, so we need to keep the joint fixed to draw moment to the joint, however, by doing so the resulting moment won't be correct (which will be negative), so we realize, in order to get the correct result, we need to modify the joint support specification.
Now, by examining the expected deflection of the full structure at the same location of the cut, we know the joint must be able to move freely in the vertical direction to allow deflection to occur, so we need to remove the restrain in the Y direction. Note, by releasing the vertical support at the cut, the moment curve is corrected as well.
The graph below shows the analyses results of the reduced model, which has the rightmost support fixed with Y fixity released, and the full structure.
Cut the structure at a point that will result in one side structure is the mirror image of the other.
Inspect the moment and deflection curves on the whole structure at the cut location to determine the support type to be specified to the joint of the cut. Consider partial release of support fixity if necessary.