A. This is a good concern and valid question, and the answer is yes, because when you over-diversify, you simply create redundancy, for example, if you might own 50 ETFs, and ends up with Apple in all 50.
On the other hand, when you try to streamline and become more efficient, you could hurt yourself too. For example, if you own a S&P 500 fund, you may wonder why do I need another fund that represent the U.S. stock market?
The reason is, S&P 500 only includes 500 companies, and it's cap-weighted which means it places more money on the biggest companies. So the performance of the bottom 450 companies makes little difference when they are dwarfed by the top 50 companies' performances. If you want to diversify, you need to own stocks from those 450 companies, plus other middle and small companies' stocks.