A double top is a classic bearish reversal pattern. After a strong uptrend, price reaches a high, pulls back, then rallies again to roughly the same level and fails. The two peaks form an “M.” The low between them is the neckline. The pattern is only confirmed when price closes below that neckline—meaning buyers failed twice at the same resistance and sellers took control. The typical measured target is the height of the pattern projected downward from the breakdown. 
Peaks don’t have to be identical; a few percent difference is common. Volume often fades on the second peak, which is another warning that momentum is weakening. ~ GROK