The S&P 500 index is wrapping up a historic two-year run of consecutive over 20% gains, a feat achieved only four times in the past 150 years. Yet, Michael Hartnett, chief investment strategist at Bank of America, says the party may not last forever.

According to Hartnett's latest Flow Show report, published Friday, the U.S. equity market is poised for another "double-digit move" in 2025 — though investors shouldn't rule out the possibility of a second wave of inflation that could derail the bull market.

The Bank of America's Investment Clock — a key barometer that shows the...