Your question is Correlation Between Perfectly Correlated Stocks. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Suppose you have three stocks in a portfolio: if stock 1 is perfectly correlated with stock 2, and stock 2 is perfectly correlated with stock 3, what is the correlation between 1 and 3?