Well that depends on how many partners you intend to have and use the dildo with. I recon it could get quite expensive to get a new one if you change partners quite frequently
I don't see why it would be of any problem to use the same one as long as you clean it properly and disinfect it and all that. However, what matters the most is what your partner thinks of it. So I would go ahead and tell the partner that I have used the dildo on someone before and ask if it bothers them. If it does, then don't use it.