Can anyone tell me what is the impact (if any) to 17-4 PH material that has already been precipitation hardened to H900 and undergoes the exact same process a second time?
The physical properties will be the same - there may be some additional dimensional contraction. What is your main concern?
Typically, when you have questions like this, you should go directly to the company which performed the initial heat treatment and ask. I am aware of many times various steel alloys go through multiple heat treatments with no particular problems.
Is the product undergoing second heat treatment bar stock or finished product? With finished product, you "may" encounter distortion, but not likely.
Incidentally, H900 merely means the product has been heated to 900 degrees Fahrenheit and held there for 90 minutes, then allowed to air cool (no quenching.)